Method for making textured multilayer optical films

ABSTRACT

Methods and apparatuses are provided for the manufacture of coextruded polymeric multilayer optical films. The multilayer optical films have an ordered arrangement of layers of two or more materials having particular layer thicknesses and a prescribed layer thickness gradient throughout the multilayer optical stack. The methods and apparatuses described allow improved control over individual layer thicknesses, layer thickness gradients, indices of refraction, interlayer adhesion, and surface characteristics of the optical films. The methods and apparatuses described are useful for making interference polarizers, mirrors, and colored films that are optically effective over diverse portions of the ultraviolet, visible, and infrared spectra.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/229,724, filed Jan. 13, 1999, which is acontinuation-in-part of U.S. patent application Ser. No. 09/006,288filed on Jan. 13, 1998, now abandoned, and incorporated herein byreference.

FIELD OF THE INVENTION

[0002] The present invention relates to processes for making polymericmultilayered films, and in particular to coextruded multilayered opticalfilms having alternating polymeric layers with differing indices ofrefraction wherein at least one of the polymers is able to develop andmaintain a large birefringence when stretched.

BACKGROUND OF THE INVENTION

[0003] The present invention relates to processes for making polymericmultilayered films, and more particularly to coextruded multilayeredoptical films having alternating polymeric layers with differing indicesof refraction. Various process have been devised for making multilayerfilm structures that have an ordered arrangement of layers of variousmaterials having particular layer thicknesses. Exemplary of thesestructures are those which produce an optical or visual effect becauseof the interaction of contiguous layers of materials having differentrefractive indices and layer thicknesses.

[0004] Multilayer films have previously been made or suggested to bemade by the use of complex coextrusion feedblocks alone, see, e.g., U.S.Pat. Nos. 3,773,882 and 3,884,606 to Schrenk, and the suggestion hasbeen made to modify such a device to permit individual layer thicknesscontrol as described in U.S. Pat. No. 3,687,589 to Schrenk. Suchmodified feedblocks could be used to make a multilayer film with adesired layer thickness gradient or distribution of layer thicknesses.These devices are very difficult and costly to manufacture, and arelimited in practical terms to making films of no more than about threehundred total layers. Moreover, these devices are complex to operate andnot easily changed over from the manufacture of one film construction toanother.

[0005] Multilayer films have also been made by a combination of afeedblock and one or more multipliers or interfacial surface generatorsin series, for example as described in U.S. Pat. Nos. 3,565,985 and3,759,647 to Schrenk et al. Such a combination of a feedblock andinterfacial surface generator is more generally applicable for producinga film having a large number of layers because of the greaterflexibility or adaptability and lesser manufacturing costs associatedwith a feedblock/ISG combination. An improved ISG for making multilayerfilms having a prescribed layer thickness gradient in the thicknesses oflayers of one or more materials from one major surface of the film to anopposing surface was described in U.S. Pat. Nos. 5,094,788 and 5,094,793to Schrenk et al. Schrenk described a method and apparatus in which afirst stream of discrete, overlapping layers is divided into a pluralityof branch streams which are redirected or repositioned and individuallysymmetrically expanded and contracted, the resistance to flow and thusthe flow rates of each of the branch streams are independently adjusted,and the branch streams are recombined in an overlapping relationship toform a second stream which has a greater number of discrete, overlappinglayers distributed in the prescribed gradient. The second stream may besymmetrically expanded and contracted as well. Multilayer films made inthis way are generally extremely sensitive to thickness changes, and itis characteristic of such films to exhibit streaks and spots ofnonuniform color. Further, the reflectivity of such films is highlydependent on the angle of incidence of light impinging on the film.Films made with the materials and processes heretofore described aregenerally not practical for uses which require uniformity ofreflectivity.

[0006] Several of the patents and applications discussed above containteachings with respect to introducing layer thickness gradients intomultilayer polymeric bodies. For example, U.S. Pat. No. 3,711,176 toSchrenk et al., teaches that it is desirable that a gradient or otherdistribution in the thicknesses of layers of one or more materials beestablished through the thickness of the film. Methods for creatinggradients include embossing the film, selective cooling of the filmduring final stretching, and the use of a rotating die to create thelayers as described in U.S. Pat. Nos. 3,195,865; 3,182,965; and3,051,452. These techniques attempted to introduce layer thicknessgradients into an already extruded film, and did not permit precisegeneration or control of the gradients. U.S. Pat. No. 3,687,589 toSchrenk et al teaches the use of a rotating or reciprocating shearproducing means to vary the volume of material entering the feed slotsof a coextrusion feedblock where the polymer streams are subdivided.Precise control of volumetric flow rates using such a device isdifficult to achieve. In U.S. Pat. No. 5,094,788, Schrenk et al teachusing variable vanes in an ISG downstream from a coextrusion die tointroduce a layer thickness gradient into a multilayer polymer meltstream. U.S. Pat. No. 5,389,324 to Lewis et al describes control of therespective flow rates of the polymeric materials in the substreams toprovide a differential in the volume of material flowing through each ofthe substreams. Because of the differential in the volume of thepolymeric materials flowing in the substreams making up the compositestream, the individual layers in the body have a gradient in thethicknesses. The flow rate is controlled either by providing atemperature differential among at least some of the substreams, causingchanges in the viscosities of the polymeric materials and therebycontrolling their flow, or the flow rate is controlled by modifying thegeometry of the passages or feed slots through which the plastifiedpolymeric materials flow in the feedblock. In this way, the pathlengths, widths, or heights of the substreams can be modified to controlthe flow rate of the polymer streams and thus the thickness of thelayers formed.

[0007] To form a multilayered film, after exiting either a feedblock ora combined feedblock/ISG, a multilayered stream typically passes into anextrusion die which is constructed so that streamlined flow ismaintained and the extruded product forms a multilayered film in whicheach layer is generally parallel to the major surface of adjacentlayers. Such an extrusion device is described in U.S. Pat. No. 3,557,265to Chisholm et al. One problem associated with microlayer extrusiontechnology has been flow instabilities which can occur when two or morepolymers are simultaneously extruded through a die. Such instabilitiesmay cause waviness and distortions at the polymer layer interfaces, andin severe cases, the layers may become intermixed and lose theirseparate identities, termed layer breakup. The importance of uniformlayers, i.e., layers having no waviness, distortions, or intermixing, isparamount in applications where the optical properties of themultilayered article are used. Even modest instabilities in processing,resulting in layer breakup in as few as 1% of the layers, may severelydetract from the reflectivity or appearance of an article. To formhighly reflective bodies or films, the total number of layer interfacesmust be increased, and as the number of layers in such articles isincreased in the coextrusion apparatus, individual layer thicknessesbecome smaller so that the breakup of even a relatively few layers cancause substantial deterioration of the optical properties on thearticle. Problems of layer breakup are especially severe formultilayered bodies in which individual layer thicknesses approach about10 μm or less adjacent to the walls of the feedblock, multiplier, orextrusion die. Flow of multiple polymer layers through the feedblock andISG typically entails both shear and extensional flow, while flowoutside of the extrusion die is shear-free extensional flow. Layerbreakup occurs inside flow channels very close to the channel wallswhere shear flow predominates, and is affected by such factors as smalllayer thickness, shear stress, interfacial tension between polymerlayers, interfacial adhesion between the polymer melt and channel walls,and various combinations of these factors.

[0008] Several potential suggestions have been made to minimize flowinstability, including increasing skin layer thickness nearest the diewall, decreasing the viscosity of the layer nearest the die wall byeither increasing temperature or switching to a lower viscosity resin,reducing the total extrusion rate, or increasing the die gap. In U.S.Pat. No. 4,540,623 to Im et al, the use of sacrificial or integral skinlayers on the order of from about 1 to 10 mils (25.4 to 254 μm) isdescribed to ease processing and to protect the surfaces from damage.These exterior skin layers are added immediately prior to the multilayerfilm exiting from the forming die or prior to layer multiplication. InU.S. Pat. No. 5,269,995 to Ramanathan et al, the use of protectiveboundary layers (PBLs) of a heat plastified extrudable thermoplasticmaterial is taught to minimize layer instabilities. These layers may beinternal to the multilayer body and/or on the external surfaces andgenerally serve to prevent layer breakup during the formation andmanipulation of the multiple layers of polymers in a coextrudedmultilayer polymeric body.

[0009] While the previous discussion applies to multilayered films ingeneral, often independent of the chemical, physical, or opticalproperties of the materials that make up the multilayered stack, byselective choice of materials and proper control of subsequentprocessing steps, multilayered films with enhanced optical or physicalproperties can be obtained. For example, U.S. Pat. Nos. 5,486,949 and5,612,820 to Schrenk et al describe the use of birefringent polymers forthe fabrication of coextruded polymeric multilayer optical films usefulas interference polarizers. The birefringent polymers can be oriented byuniaxial or biaxial stretching to orient the polymer on a molecularlevel such as taught in U.S. Pat. No. 4,525,413 to Rogers et al. inorder to obtain desired matches or mismatches of the in-plane refractiveindices to reflect or transmit desired polarizations. Further, in U.S.patent application Ser. No. 08/402,041 to Jonza et al the use ofbirefringent materials useful for making interference polarizers andmirrors is described in which control of the relationships between thein-plane and out-of-plane indices of refraction gives coextrudedpolymeric multilayer optical films with improved optical properties atnon-normal angles.

[0010] Recent developments in materials available for use in makingpolymeric multilayer optical films, and new uses for optical films whichrequire improved control of layer thickness and/or the relationshipsbetween the in-plane and out-of-plane indices of refraction, have beenidentified. Processes described heretofore typically are not able toexploit the potential of the new resins available and do not provide therequired degree of versatility and control over absolute layerthickness, layer thickness gradients, indices of refraction,orientation, and interlayer adhesion that is needed for the routinemanufacture of many of these films. Accordingly, there exists a need inthe art for an improved process for making coextruded polymericmultilayer optical films with greater versatility and enhanced controlover several steps in the manufacturing process.

SUMMARY OF THE INVENTION

[0011] The present invention relates to methods and apparatuses formaking multilayered optical films.

[0012] In brief summary, a useful feedblock useful for making amultilayer optical film of the invention comprises: (a) a gradient platecomprising at least first and second flow channels, wherein at least oneof the flow channel has a cross-sectional area that changes from a firstposition to a second position along the flow channel; (b) a feeder tubeplate having a first plurality of conduits in fluid communication withthe first flow channel and a second plurality of conduits in fluidcommunication with the second flow channel, each conduit feeding its ownrespective slot die, each conduit having a first end and a second end,the first end of the conduits being in fluid communication with the flowchannels, and the second end of the conduits being in fluidcommunication with the slot die; and (c) an axial rod heater locatedproximal to said conduits.

[0013] In brief summary, a method for making a multilayered optical filmcomprises the steps of: (a) providing at least a first and a secondstream of resin; (b) dividing the first and the second streams into aplurality of layers using a feedblock comprising: (i) a gradient platecomprising first and second flow channels, where the first channel has across-sectional area that changes from a first position to a secondposition along the flow channel; (ii) a feeder tube plate having a firstplurality of conduits in fluid communication with the first flow channeland a second plurality of conduits in fluid communication with thesecond flow channel, each conduit feeding its own respective slot die,each conduit having a first end and a second end, the first end of theconduits being in fluid communication with the flow channels, and thesecond end of the conduits being in fluid communication with the slotdie; and (c) an axial rod heater located proximal to said conduits (c)passing the composite stream through an extrusion die to form amultilayer web in which each layer is generally parallel to the majorsurface of adjacent layers; and (d) casting the multilayer web onto acasting roll to form a cast multilayer film.

[0014] In brief summary, a method of making a textured multilayeroptical film comprises the steps of: (a) providing at least a first anda second stream of resin; (b) dividing the first and the second streamsinto a plurality of layers such that the layers of the first stream areinterleaved with the layers of the second stream to yield a compositestream; (c) passing the composite stream through an extrusion die toform a multilayer web in which each layer is generally parallel to themajor surface of adjacent layers; (d) casting the multilayer web onto acasting roll; and (e) contacting the multilayer web by a micro-embossingroll to form a cast multilayer film.

[0015] In yet another method of making a multilayer optical film, themethod comprises the steps of: (a) providing at least a first and asecond stream of resin, wherein the first stream of resin is a copolymerof polyethylene naphthalate (coPEN) and the second stream of resin ispolymethyl methacrylate (PMMA), (b) dividing the first and the secondstreams into a plurality of layers such that the layers of the firststream are interleaved with the layers of the second stream to yield acomposite stream; (c) coextruding the composite stream through a die toform a multilayer web wherein each layer is generally parallel to themajor surface of adjacent layers, wherein the coPEN and PMMA resins arecoextruded at a melt temperature of about 260° C., and wherein thebirefringence of the coPEN resin is reduced by about 0.02 units or lesscompared to the birefringence of a homopolymer PEN resin for a givendraw ratio; and (d) casting the multilayer web onto a casting roll toform a cast multilayer film.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention may be more completely understood in considerationof the following detailed description of various embodiments of theinvention in connection with the accompanying drawings, in which:

[0017]FIG. 1 is a schematic drawing illustrating the general processuseful for the coextrusion of multilayered optical films made inaccordance with the present invention;

[0018]FIG. 2 is a schematic diagram a portion of the apparatuses usefulin the process of making a multilayered optical film of the presentinvention; and

[0019]FIG. 3 is a schematic diagram of a feedblock useful in the processof making a multilayered optical film of the present invention.

[0020] These figures are idealized, are not to scale, and are intendedto be merely illustrative and non-limiting.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Various process considerations are important in making highquality polymeric multilayer optical films and other optical devices inaccordance with the present invention. Such optical films include, butare not limited to, interference polarizers, mirrors, colored films, andcombinations thereof. The films are optically effective over diverseportions of the ultraviolet, visible, and infrared spectra. Ofparticular interest are coextruded polymeric multilayer optical filmshaving one or more layers that are birefringent in nature. The processconditions used to make each depends in part on (1) the particular resinsystem used and (2) the desired optical properties of the final film.

[0022] A preferred method of making the multilayer film of the presentinvention is illustrated schematically in FIG. 1. Materials 100 and 102,selected to have suitably different optical properties, are heated abovetheir melting and/or glass transition temperatures and fed into amultilayer feedblock 104. Typically, melting and initial feeding isaccomplished using an extruder for each material. For example, material100 can be fed into an extruder 101 while material 102 can be fed intoan extruder 103. Exiting from the feedblock 104 is a multilayer flowstream 105. A layer multiplier 106 splits the multilayer flow stream,and then redirects and “stacks” one stream atop the second to multiplythe number of layers extruded. An asymmetric multiplier, when used withextrusion equipment that introduces layer thickness deviationsthroughout the stack, may broaden the distribution of layer thicknessesso as to enable the multilayer film to have layer pairs corresponding toa desired portion of the visible spectrum of light, and provide adesired layer thickness gradient. If desired, skin layers 111 may beintroduced into the multilayer optical film by feeding resin 108 (forskin layers) to a skin layer feedblock 110.

[0023] The multilayer feedblock feeds a film extrusion die 112.Feedblocks useful in the manufacture of the present invention aredescribed in, for example, U.S. Pat. No. 3,773,882 (Schrenk) and U.S.Pat. No. 3,884,606 (Schrenk), the contents of which are incorporated byreference herein. As an example, the extrusion temperature may beapproximately 295° C., and the feed rate approximately 10-150 kg/hourfor each material. It is desirable in most cases to have skin layers 111flowing on the upper and lower surfaces of the film as it goes throughthe feedblock and die. These layers serve to dissipate the large stressgradient found near the wall, leading to smoother extrusion of theoptical layers. Typical extrusion rates for each skin layer would be2-50 kg/hr (1-40% of the total throughput). The skin material can be thesame material as one of the optical layers or be a different material.An extrudate leaving the die is typically in a melt form.

[0024] The extrudate is cooled on a casting wheel 116, which rotatespast pinning wire 114. The pinning wire pins the extrudate to thecasting wheel. To achieve a clear film over a broad range of angles, onecan make the film thicker by running the casting wheel at a slow speed,which moves the reflecting band towards longer wavelengths. The film isoriented by stretching at ratios determined by the desired optical andmechanical properties. Longitudinal stretching can be done by pull rolls118. Transverse stretching can be done in a tenter oven 120. If desired,the film can be bi-axially oriented simultaneously. Stretch ratios ofapproximately 3-4 to 1 are preferred, although ratios as small as 2 to 1and as large as 6 to 1 may also be appropriate for a given film. Stretchtemperatures will depend on the type of birefringent polymer used, but2° to 33° C. (5° to 60° F.) above its glass transition temperature wouldgenerally be an appropriate range. The film is typically heat set in thelast two zones 122 of the tenter oven to impart the maximumcrystallinity in the film and reduce its shrinkage. Employing a heat settemperature as high as possible without causing film breakage in thetenter reduces the shrinkage during a heated embossing step. A reductionin the width of the tenter rails by about 1-4% also serves to reducefilm shrinkage. If the film is not heat set, heat shrink properties aremaximized, which may be desirable in some security packagingapplications. The film can be collected on windup roll 124.

[0025] In some applications, it may be desirable to use more than twodifferent polymers in the optical layers of the multilayer film. In sucha case, additional resin streams can be fed using similar means to resinstreams 100 and 102. A feedblock appropriate for distributing more thantwo layer types analogous to the feedblock 104 could be used.

[0026]FIG. 2 is a schematic representation of a portion of a typicalset-up useful for the practice of the present invention. Feedblock 200has four sections: a gradient plate 202, a feeder tube plate 204, anoptional slot plate 206, and optional compression section 208. The slotplate houses a plurality of individual slots, which is a part of theslot die (not shown). Alternatively, the slots can be a part of thefeeder tube plate. The compression section is typically located in thefeedblock, although it does not need to be. Adjacent to the feedblock isa unit 210 useful for the introduction of protective boundary layers.Although multipliers 212 and 214 are shown, it is within the scope ofthis invention to use no multipliers or at least one multiplier toincrease the number of layers in the multilayer optical film. A unit 216is useful for the introduction of skin layers, if desired. A filmcasting die 218 begins the formation of the multilayer film. Extrudateexiting the casting die is allowed to contact a casting wheel 220. Thecasting wheel is typically cooled to quench the extrudate and form afilm. Additional processing, such as drawing, orienting, andheat-setting the inventive multilayer film can also be done.

[0027] The above description is intended to provide an overview of themethod and apparatus encompassed within the present invention. It shouldbe understood, however, that the intention is not to limit the inventionto the particular embodiments described, but to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the appended claims.

[0028] Material Selection

[0029] A variety of polymer materials suitable for use in the presentinvention have been taught for use in making coextruded multilayeroptical films. For example, the polymer materials listed and describedin U.S. Pat. Nos. 4,937,134, 5,103,337, 5,1225,448,404, 5,540,978, and5,568,316 to Schrenk et al., and in U.S. Pat. Nos. 5,122,905, 5,122,906,and 5,126,880 to Wheatley and Schrenk are useful for making multilayeroptical films according to the present invention. Of special interestare birefringent polymers such as those described in U.S. Pat. Nos.5,486,949 and 5,612,820 to Schrenk et al; in U.S. patent applicationSer. No. 08/402,041 to Jonza et al.; and in U.S. patent application Ser.No. 09/006,601 entitled “Modified Copolyesters and Improved MultilayerReflective Films” filed Jan. 13, 1998, are of which are incorporated byreference. Regarding the preferred materials from which the films are tobe made, there are several conditions which should be met to make themultilayer optical films of this invention. First, these films shouldconsist of at least two distinguishable polymers. The number of polymersis not limited, and three or more polymers may be advantageously used inparticular films. Second, at least one of the two required polymers,commonly referred to as the “first polymer,” preferably has a stressoptical coefficient having a large absolute value. In other words, thefirst polymer preferably develops a large birefringence when stretched.Depending on the application of the multilayer film, the birefringencemay be developed between two orthogonal directions in the plane of thefilm, between one or more in-plane directions and the directionperpendicular to the film plane, or a combination of these. In thespecial case that the isotropic indices are widely separated, thepreference for large birefringence in the first polymer may be relaxed,although birefringence is still usually desirable. Such special casesmay arise in the selection of polymers for mirror films and forpolarizer films formed using a biaxial process, which draws the film intwo orthogonal in-plane directions. Third, the first polymer should becapable of maintaining birefringence after stretching, so that thedesired optical properties are imparted to the finished film. Fourth,the other required polymer, commonly referred to as the “secondpolymer,” should be chosen so that in the finished film, its refractiveindex, in at least one direction, differs significantly from the indexof refraction of the first polymer in the same direction. Becausepolymeric materials are typically dispersive, that is, the refractiveindices vary with wavelength, these conditions must be considered interms of a particular spectral bandwidth of interest.

[0030] Other aspects of polymer selection depend on specificapplications. For polarizing films, it is advantageous for thedifference in the index of refraction of the first and second polymersin one film-plane direction to differ significantly in the finishedfilm, while the difference in the orthogonal film-plane index isminimized. If the first polymer has a large refractive index whenisotropic, and is positively birefringent (that is, its refractive indexincreases in the direction of stretching), the second polymer willtypically be chosen to have a matching refractive index, afterprocessing, in the planar direction orthogonal to the stretchingdirection, and a refractive index in the direction of stretching whichis as low as possible. Conversely, if the first polymer has a smallrefractive index when isotropic, and is negatively birefringent, thesecond polymer will typically be chosen to have a matching refractiveindex, after processing, in the planar direction orthogonal to thestretching direction, and a refractive index in the direction ofstretching which is as high as possible.

[0031] Alternatively, it is possible to select a first polymer which ispositively birefringent and has an intermediate or low refractive indexwhen isotropic, or one which is negatively birefringent and has anintermediate or high refractive index when isotropic. In these cases,the second polymer may typically be chosen so that, after processing,its refractive index will match that of the first polymer in either thestretching direction or the planar direction orthogonal to stretching.Further, the second polymer will typically be chosen such that thedifference in index of refraction in the remaining planar direction ismaximized, regardless of whether this is best accomplished by a very lowor very high index of refraction in that direction.

[0032] There are several means to achieve the combination of planarindex matching in one direction and mismatching in the orthogonaldirection. For example, one can select a first polymer which developssignificant birefringence when stretched, select a second polymer whichdevelops little or no birefringence when stretched, and to stretch theresulting film in only one planar direction. In another method, thesecond polymer can be selected from among those which developbirefringence in the sense opposite to that of the first polymer(negative—positive or positive—negative). Another method involvesselecting both first and second polymers which are capable of developingbirefringence when stretched, but to stretch the multilayer film in twoorthogonal planar directions. This latter method involves selectingprocess conditions (such as temperatures, stretch rates, post-stretchrelaxation, and the like) that result in the development of unequallevels of orientation in the two stretching directions for the first andsecond polymers, such that one in-plane index is approximately matchedto that of the first polymer, and the orthogonal in-plane index issignificantly mismatched to that of the first polymer. For example,conditions may be chosen such that the first polymer has a biaxiallyoriented character in the finished film, while the second polymer has apredominantly uniaxially oriented character in the finished film.

[0033] The foregoing discussion for polarizing film is meant to beexemplary. It will be understood that combinations of these and othertechniques may be used to achieve the index mismatch in one in-planedirection and relative index matching in the orthogonal planardirection.

[0034] Different considerations apply to a reflective, or mirror, film.Provided that the film is not meant to have some polarizing propertiesas well, refractive index criteria apply equally to any direction in thefilm plane. Thus, typical for the indices for any given layer inorthogonal in-plane directions to be nearly equal. It is advantageous,however, for the film-plane indices of the first polymer to differ asgreatly as possible from the film-plane indices of the second polymer.For this reason, if the first polymer has a high index of refractionwhen isotropic, it is advantageous that it also be positivelybirefringent. Likewise, if the first polymer has a low index ofrefraction when isotropic, it is advantageous that it also be negativelybirefringent. The second polymer advantageously develops little or nobirefringence when stretched, or develops birefringence of the oppositesense (positive—negative or negative—positive), such that its film-planerefractive indices differ as much as possible from those of the firstpolymer in the finished film. These criteria may be combinedappropriately with those listed above for polarizing films if a mirrorfilm is meant to have some degree of polarizing properties as well.

[0035] Colored films can be regarded as special cases of mirror andpolarizing films. Thus, the same criteria outlined above apply. Theperceived color is a result of reflection or polarization over one ormore specific bandwidths of the spectrum. The bandwidths over which amultilayer film of the current invention is effective will be determinedprimarily by the distribution of layer thicknesses used in the opticalstack(s), but consideration must also be given to the wavelengthdependence, or dispersion, of the refractive indices of the first andsecond polymers. It will be understood that the same rules apply to theinfrared and ultraviolet wavelengths as to the visible colors.

[0036] Absorbance is another consideration. For most applications, it isadvantageous for neither the first nor the second polymer to have anyabsorbance bands within the bandwidth of interest for the film inquestion. Thus, all incident light within the bandwidth is eitherreflected or transmitted. However, for some applications, it may beuseful for one or both of the first and second polymer to absorbspecific wavelengths, either totally or in part.

[0037] Although many polymers may be chosen as the first polymer,certain of the polyesters have the capability for particularly largebirefringence. Among these, polyethylene 2,6-naphthalate (PEN) isfrequently chosen as a first polymer for films of the present invention.It has a very large positive stress optical coefficient, retainsbirefringence effectively after stretching, and has little or noabsorbance within the visible range. It also has a large index ofrefraction in the isotropic state. Its refractive index for polarizedincident light of 550 nm wavelength increases when the plane ofpolarization is parallel to the stretch direction from about 1.64 to ashigh as about 1.9. Its birefringence can be increased by increasing itsmolecular orientation which, in turn, may be increased by stretching togreater stretch ratios with other stretching conditions held fixed.

[0038] Other semicrystalline naphthalene dicarboxylic polyesters arealso suitable as first polymers. Polybutylene 2,6-Naphthalate (PBN) isan example. These polymers may be homopolymers or copolymers, providedthat the use of comonomers does not substantially impair the stressoptical coefficient or retention of birefringence after stretching. Theterm “PEN” herein will be understood to include copolymers of PENmeeting these restrictions. In practice, these restrictions imposes anupper limit on the comonomer content, the exact value of which will varywith the choice of comonomer(s) employed. Some compromise in theseproperties may be accepted, however, if comonomer incorporation resultsin improvement of other properties. Such properties include but are notlimited to improved interlayer adhesion, lower melting point (resultingin lower extrusion temperature), better rheological matching to otherpolymers in the film, and advantageous shifts in the process window forstretching due to change in the glass transition temperature.

[0039] Suitable comonomers for use in PEN, PBN or the like may be of thediol or dicarboxylic acid or ester type. Dicarboxylic acid comonomersinclude but are not limited to terephthalic acid, isophthalic acid,phthalic acid, all isomeric naphthalenedicarboxylic acids (2,6-, 1,2-,1,3-, 1,4-, 1,5-, 1,6-, 1,7-, 1,8-, 2,3-, 2,4-, 2,5-, 2,7-, and 2,8-),bibenzoic acids such as 4,4′-biphenyl dicarboxylic acid and its isomers,trans-4,4′-stilbene dicarboxylic acid and its isomers, 4,4′-diphenylether dicarboxylic acid and its isomers, 4,4′-diphenylsulfonedicarboxylic acid and its isomers, 4,4′-benzophenone dicarboxylic acidand its isomers, halogenated aromatic dicarboxylic acids such as2-chloroterephthalic acid and 2,5-dichloroterephthalic acid, othersubstituted aromatic dicarboxylic acids such as tertiary butylisophthalic acid and sodium sulfonated isophthalic acid, cycloalkanedicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid and itsisomers and 2,6-decahydronaphthalene dicarboxylic acid and its isomers,bi- or multi-cyclic dicarboxylic acids (such as the various isomericnorbornane and norbornene dicarboxylic acids, adamantane dicarboxylicacids, and bicyclo-octane dicarboxylic acids), alkane dicarboxylic acids(such as sebacic acid, adipic acid, oxalic acid, malonic acid, succinicacid, glutaric acid, azelaic acid, and dodecane dicarboxylic acid.), andany of the isomeric dicarboxylic acids of the fused-ring aromatichydrocarbons (such as indene, anthracene, pheneanthrene, benzonaphthene,fluorene and the like). Alternatively, alkyl esters of these monomers,such as dimethyl terephthalate, may be used.

[0040] Suitable diol comonomers include but are not limited to linear orbranched alkane diols or glycols (such as ethylene glycol, propanediolssuch as trimethylene glycol, butanediols such as tetramethylene glycol,pentanediols such as neopentyl glycol, hexanediols,2,2,4-trimethyl-1,3-pentanediol and higher diols), ether glycols (suchas diethylene glycol, triethylene glycol, and polyethylene glycol),chain-ester diols such as3-hydroxy-2,2-dimethylpropyl-3-hydroxy-2,2-dimethyl propanoate,cycloalkane glycols such as 1,4-cyclohexanedimethanol and its isomersand 1,4-cyclohexanediol and its isomers, bi- or multicyclic diols (suchas the various isomeric tricyclodecane dimethanols, norbornanedimethanols, norbornene dimethanols, and bicyclo-octane dimethanols),aromatic glycols (such as 1,4-benzenedimethanol and its isomers,1,4-benzenediol and its isomers, bisphenols such as bisphenol A,2,2′-dihydroxy biphenyl and its isomers, 4,4′-dihydroxymethyl biphenyland its isomers, and 1,3-bis(2-hydroxyethoxy)benzene and its isomers),and lower alkyl ethers or diethers of these diols, such as dimethyl ordiethyl diols.

[0041] Tri- or polyfunctional comonomers, which can serve to impart abranched structure to the polyester molecules, can also be used. Theymay be of either the carboxylic acid, ester, hydroxy or ether types.Examples include, but are not limited to, trimellitic acid and itsesters, trimethylol propane, and pentaerythritol.

[0042] Also suitable as comonomers are monomers of mixed functionality,including hydroxycarboxylic acids such as parahydroxybenzoic acid and6-hydroxy-2-naphthalenecarboxylic acid, and their isomers, and tri- orpolyfunctional comonomers of mixed functionality such as5-hydroxyisophthalic acid and the like.

[0043] Polyethylene terephthalate (PET) is another material thatexhibits a significant positive stress optical coefficient, retainsbirefringence effectively after stretching, and has little or noabsorbance within the visible range. Thus, it and its high PET-contentcopolymers employing comonomers listed above may also be used as firstpolymers in some applications of the current invention.

[0044] When a naphthalene dicarboxylic polyester such as PEN or PBN ischosen as first polymer, there are several approaches which may be takento the selection of a second polymer. One preferred approach for someapplications is to select a naphthalene dicarboxylic copolyester (coPEN)formulated so as to develop significantly less or no birefringence whenstretched. This can be accomplished by choosing comonomers and theirconcentrations in the copolymer such that crystallizability of the coPENis eliminated or greatly reduced. One typical formulation employs as thedicarboxylic acid or ester components dimethyl naphthalate at from about20 mole percent to about 80 mole percent and dimethyl terephthalate ordimethyl isophthalate at from about 20 mole percent to about 80 molepercent, and employs ethylene glycol as diol component. Of course, thecorresponding dicarboxylic acids may be used instead of the esters. Thenumber of comonomers which can be employed in the formulation of a coPENsecond polymer is not limited. Suitable comonomers for a coPEN secondpolymer include but are not limited to all of the comonomers listedabove as suitable PEN comonomers, including the acid, ester, hydroxy,ether, tri- or polyfunctional, and mixed functionality types.

[0045] Often it is useful to predict the isotropic refractive index of acoPEN second polymer. A volume average of the refractive indices of themonomers to be employed has been found to be a suitable guide. Similartechniques well-known in the art can be used to estimate glasstransition temperatures for coPEN second polymers from the glasstransitions of the homopolymers of the monomers to be employed.

[0046] In addition, polycarbonates having a glass transition temperaturecompatible with that of PEN and having a refractive index similar to theisotropic refractive index of PEN are also useful as second polymers.Polyesters, copolyesters, polycarbonates, and copolycarbonates may alsobe fed together to an extruder and transesterified into new suitablecopolymeric second polymers.

[0047] It is not required that the second polymer be a copolyester orcopolycarbonate. Vinyl polymers and copolymers made from monomers suchas vinyl naphthalenes, styrenes, ethylene, maleic anhydride, acrylates,acetates, and methacrylates may be employed. Condensation polymers otherthan polyesters and polycarbonates may also be used. Examples include:polysulfones, polyamides, polyurethanes, polyamic acids, and polyimides.Naphthalene groups and halogens such as chlorine, bromine and iodine areuseful for increasing the refractive index of the second polymer to adesired level. Acrylate groups and fluorine are particularly useful indecreasing refractive index when this is desired.

[0048] It will be understood from the foregoing discussion that thechoice of a second polymer is dependent not only on the intendedapplication of the multilayer optical film in question, but also on thechoice made for the first polymer, and the processing conditionsemployed in stretching. Suitable second polymer materials include butare not limited to polyethylene naphthalate (PEN) and isomers thereof(such as 2,6-, 1,4-, 1,5-, 2,7-, and 2,3-PEN), polyalkyleneterephthalates (such as polyethylene terephthalate, polybutyleneterephthalate, and poly-1,4-cyclohexanedimethylene terephthalate), otherpolyesters, polycarbonates, polyarylates, polyamides (such as nylon 6,nylon 11, nylon 12, nylon 4/6, nylon 6/6, nylon 6/9, nylon 6/10, nylon6/12, and nylon 6/T), polyimides (including thermoplastic polyimides andpolyacrylic imides), polyamide-imides, polyether-amides,polyetherimides, polyaryl ethers (such as polyphenylene ether and thering-substituted polyphenylene oxides), polyarylether ketones such aspolyetheretherketone (“PEEK”), aliphatic polyketones (such as copolymersand terpolymers of ethylene and/or propylene with carbon dioxide),polyphenylene sulfide, polysulfones (includine polyethersulfones andpolyaryl sulfones), atactic polystyrene, syndiotactic polystyrene(“sPS”) and its derivatives (such as syndiotactic poly-alpha-methylstyrene and syndiotactic polydichlorostyrene), blends of any of thesepolystyrenes (with each other or with other polymers, such aspolyphenylene oxides), copolymers of any of these polystyrenes (such asstyrene-butadiene copolymers, styrene-acrylonitrile copolymers, andacrylonitrile-butadiene-styrene terpolymers), polyacrylates (such aspolymethyl acrylate, polyethyl acrylate, and polybutyl acrylate),polymethacrylates (such as polymethyl methacrylate, polyethylmethacrylate, polypropyl methacrylate, and polyisobutyl methacrylate),cellulose derivatives (such as ethyl cellulose, cellulose acetate,cellulose propionate, cellulose acetate butyrate, and cellulosenitrate), polyalkylene polymers (such as polyethylene, polypropylene,polybutylene, polyisobutylene, and poly(4-methyl)pentene), fluorinatedpolymers and copolymers (such as polytetrafluoroethylene,polytrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride,fluorinated ethylene-propylene copolymers, perfluoroalkoxy resins,polychlorotrifluoroethylene, polyethylene-co-trifluoroethylene,polyethylene-co-chlorotrifluoroethylene), chlorinated polymers (such aspolyvinylidene chloride and polyvinyl chloride), polyacrylonitrile,polyvinylacetate, polyethers (such as polyoxymethylene and polyethyleneoxide), ionomeric resins, elastomers (such as polybutadiene,polyisoprene, and neoprene), silicone resins, epoxy resins, andpolyurethanes.

[0049] Also suitable are copolymers, such as the copolymers of PENdiscussed above as well as any other non-naphthalene group-containingcopolyesters which may be formulated from the above lists of suitablepolyester comonomers for PEN. In some applications, especially when PETserves as the first polymer, copolyesters based on PET and comonomersfrom said lists above (coPETs) are especially suitable. In addition,either first or second polymers may consist of miscible or immiscibleblends of two or more of the above-described polymers or copolymers(such as blends of sPS and atactic polystyrene, or of PEN and sPS). ThecoPENs and coPETs described may be synthesized directly, or may beformulated as a blend of pellets where at least one component is apolymer based on naphthalene dicarboxylic acid or terephthalic acid andother components are polycarbonates or other polyesters, such as a PET,a PEN, a coPET, or a co-PEN.

[0050] Another preferred family of materials for the second polymer forsome applications are the syndiotactic vinyl aromatic polymers, such assyndiotactic polystyrene. Syndiotactic vinyl aromatic polymers useful inthe current invention include poly(styrene), poly(alkyl styrene)s, poly(aryl styrene)s, poly(styrene halide)s, poly(alkoxy styrene)s,poly(vinyl ester benzoate), poly(vinyl naphthalene), poly(vinylstyrene),and poly(acenaphthalene), as well as the hydrogenated polymers andmixtures or copolymers containing these structural units. Examples ofpoly(alkyl styrene)s include the isomers of the following: poly(methylstyrene), poly(ethyl styrene), poly(propyl styrene), and poly(butylstyrene). Examples of poly(aryl styrene)s include the isomers ofpoly(phenyl styrene). As for the poly(styrene halide)s, examples includethe isomers of the following: poly(chlorostyrene), poly(bromostyrene),and poly(fluorostyrene). Examples of poly(alkoxy styrene)s include theisomers of the following: poly(methoxy styrene) and poly(ethoxystyrene). Among these examples, particularly preferable styrene grouppolymers, are: polystyrene, poly(p-methyl styrene), poly(m-methylstyrene), poly(p-tertiary butyl styrene), poly(p-chlorostyrene),poly(m-chloro styrene), poly(p-fluoro styrene), and copolymers ofstyrene and p-methyl styrene.

[0051] Furthermore, comonomers may be used to make syndiotactic vinylaromatic group copolymers. In addition to the monomers for thehomopolymers listed above in defining the syndiotactic vinyl aromaticpolymers group, suitable comonomers include olefin monomers (such asethylene, propylene, butenes, pentenes, hexenes, octenes or decenes),diene monomers (such as butadiene and isoprene), and polar vinylmonomers (such as cyclic diene monomers, methyl methacrylate, maleicacid anhydride, or acrylonitrile).

[0052] The syndiotactic vinyl aromatic copolymers of the presentinvention may be block copolymers, random copolymers, or alternatingcopolymers.

[0053] The syndiotactic vinyl aromatic polymers and copolymers referredto in this invention generally have syndiotacticity of higher than 75%or more, as determined by carbon-13 nuclear magnetic resonance.Preferably, the degree of syndiotacticity is higher than 85% racemicdiad, or higher than 30%, or more preferably, higher than 50%, racemicpentad.

[0054] In addition, although there are no particular restrictionsregarding the molecular weight of these syndiotactic vinyl aromaticpolymers and copolymers, preferably, the weight average molecular weightis greater than 10,000 and less than 1,000,000, and more preferably,greater than 50,000 and less than 800,000.

[0055] The syndiotactic vinyl aromatic polymers and copolymers may alsobe used in the form of polymer blends with, for instance, vinyl aromaticgroup polymers with atactic structures, vinyl aromatic group polymerswith isotactic structures, and any other polymers that are miscible withthe vinyl aromatic polymers. For example, polyphenylene ethers show goodmiscibility with many of the previous described vinyl aromatic grouppolymers.

[0056] When a polarizing film is made using a process with predominantlyuniaxial stretching, particularly preferred combinations of polymers foroptical layers include PEN/coPEN, PET/coPET, PEN/sPS, PET/sPS,PEN/Eastar,™ and PET/Eastar,™ where “coPEN” refers to a copolymer orblend based upon naphthalene dicarboxylic acid (as described above) andEastar™ is a polyester or copolyester (believed to comprisecyclohexanedimethylene diol units and terephthalate units) commerciallyavailable from Eastman Chemical Co. When a polarizing film is to be madeby manipulating the process conditions of a biaxial stretching process,particularly preferred combinations of polymers for optical layersinclude PEN/coPEN, PEN/PET, PEN/PBT, PEN/PETG and PEN/PETcoPBT, where“PBT” refers to polybutylene terephthalate, “PETG” refers to a copolymerof PET employing a second glycol (usually cyclohexanedimethanol), and“PETcoPBT” refers to a copolyester of terephthalic acid or an esterthereof with a mixture of ethylene glycol and 1,4-butanediol.

[0057] Particularly preferred combinations of polymers for opticallayers in the case of mirrors or colored films include PEN/PMMA,PET/PMMA, PEN/Ecdel,™ PET/Ecdel,™ PEN/sPS, PET/sPS, PEN/coPET, PEN/PETG,and PEN/THV,™ where “PMMA” refers to polymethyl methacrylate, Ecdel™ isa thermoplastic polyester or copolyester (believed to comprisecyclohexanedicarboxylate units, polytetramethylene ether glycol units,and cyclohexanedimethanol units) commercially available from EastmanChemical Co., “coPET” refers to a copolymer or blend based uponterephthalic acid (as described above), “PETG” refers to a copolymer ofPET employing a second glycol (usually cyclohexanedimethanol), and THV™is a fluoropolymer commercially available from 3M Co.

[0058] For mirror films, a match of the refractive indices of the firstpolymer and second polymer in the direction normal to the film plane issometimes preferred, because it provides for constant reflectance withrespect to the angle of incident light (that is, there is no Brewster'sangle). For example, at a specific wavelength, the in-plane refractiveindices might be 1.76 for biaxially oriented PEN, while the filmplane-normal refractive index might fall to 1.49. When PMMA is used asthe second polymer in the multilayer construction, its refractive indexat the same wavelength, in all three directions, might be 1.495. Anotherexample is the PET/Ecdel™ system, in which the analogous indices mightbe 1.66 and 1.51 for PET, while the isotropic index of Ecdel™ might be1.52. The crucial property is that the normal-to-plane index for onematerial must be closer to the in-plane indices of the other materialthan to its own in-plane indices.

[0059] In other embodiments, a deliberate mismatching of thenormal-to-plane refractive index is desirable. Some examples includethose involving three or more polymeric layers in the optical stack inwhich a deliberate mismatch in the normal-to-plane index is desirableopposite in sign to the index mismatch in one of the in-planedirections. It is sometimes preferred for the multilayer optical filmsof the current invention to consist of more than two distinguishablepolymers. A third or subsequent polymer might be fruitfully employed asan adhesion-promoting layer between the first polymer and the secondpolymer within an optical stack, as an additional component in a stackfor optical purposes, as a protective boundary layer between opticalstacks, as a skin layer, as a functional coating, or for any otherpurpose. As such, the composition of a third or subsequent polymer, ifany, is not limited. Some preferred multicomponent constructions aredescribed in U.S. patent application Ser. No. 09/006,118 filed Jan. 13,1998 entitled “Multicomponent Optical Body,” the contents of which areherein incorporated by reference.

[0060] Process Considerations

[0061] The process used for making the coextruded polymeric multilayeroptical films of the present invention will vary depending on the resinmaterials selected and the optical properties desired in the finishedfilm product.

[0062] Moisture sensitive resins should be dried before or duringextrusion to prevent degradation. The drying can be done by any meansknown in the art. One well-known means employs ovens or moresophisticated heated vacuum and/or desiccant hopper-dryers to dry resinprior to its being fed to an extruder. Another means employs avacuum-vented twin-screw extruder to remove moisture from the resinwhile it is being extruded. Drying time and temperature should belimited to prevent thermal degradation or sticking during hopper-dryeror oven drying. In addition, resins coextruded with moisture sensitiveresins should be dried to prevent damage to the moisture sensitivecoextruded resin from moisture carried by the other resin.

[0063] Extrusion conditions are chosen to adequately feed, melt, mix andpump the polymer resin feed streams in a continuous and stable manner.Final melt stream temperatures are chosen within a range which avoidsfreezing, crystallization or unduly high pressure drops at the low endof the temperature range and which avoids degradation at the high end ofthe temperature range. For example, polyethylene naphthalate (PEN) isdried for 8 hours at 135° C. and then vacuum fed to an extruder with afinal zone temperature, or melt temperature, ranging preferably between270° C. and 300° C. and more preferably between 275° C. and 290° C.

[0064] It is often preferable for all polymers entering the multilayerfeedblock to be at the same or very similar melt temperatures. This mayrequire process compromise if two polymers, whose ideal melt processingtemperatures do not match, are to be coextruded. For example, polymethylmethacrylate (PMMA) is typically extruded at a temperature below about250° C. Applicants have found, however, that PMMA can be coextruded withPEN using PMMA melt temperatures as high as 275° C., provide that designconsiderations are made in the PMMA melt train to minimize the potentialfor stagnation points in the flow, and to hold to a minimum the overallresidence time in the melt of the PMMA. Another technique found to beuseful in this regard is to start up the PMMA melt train at the moreconventional processing temperatures, and then to raise the melt traintemperatures to the higher, PEN-compatible temperatures only whenwell-developed flow through the entire process has been attained.

[0065] Conversely, the PEN processing temperature may be reduced so asto match it to the typical melt processing temperatures for PMMA. Thus,it has also been found that the melting point, and hence, the processingtemperature, of PEN may be reduced by the addition of comonomers intothe PEN polymer with only a very slight accompanying reduction of theability of the PEN to develop birefringence upon drawing. For example, aPEN copolymer made using DiMethyl Isophthalate (DMI) in place of 3 mol %of the 2,6-DiMethyl Naphthalate (DMN) monomer has been found to have areduction in birefringence of only 0.02 units, and a reduction of glasstransition temperature of only about 4 or 5° C., while the meltprocessing temperature is reduced by 15° C. Small amounts of DiMethylTerephthalate (DMT) or other diacid or diol comonomers may also beuseful in this regard. Esters or diesters of the diacid comonomers mayalso be used. The advantages of adding comonomers into the PEN polymerare more fully described in U.S. patent applications Ser. No. 09/006,601entitled “Modified Copolyesters and Improved Multilayer Reflective Film”and Ser. No. 09/006,468 entitled “Optical Device with a DichroicPolarizer and Multilayer Optical Film,” both filed on filed Jan. 13,1998, the contents of which are incorporated herein by reference.

[0066] It will be evident to one skilled in the art that combinations ofPEN process temperature reduction through copolymerization and PMMA melttemperature elevation via process design could be usefully employed, ascould the combination of one, the other, or both techniques with stillother techniques. Likewise, similar techniques could be employed forequal-temperature coextrusion of PEN with polymers other than PMMA, PMMAwith polymers other than PEN, or combinations including neither of thetwo exemplary polymers.

[0067] Following extrusion, the melt streams are then filtered to removeundesirable particles and gels. Primary and secondary filters known inthe art of polyester film manufacture may be used, with mesh sizes inthe 1-30 micrometer range. While the prior art indicates the importanceof such filtration to film cleanliness and surface properties, itssignificance in the present invention extends to layer uniformity aswell. Each melt stream is then conveyed through a neck tube into a gearpump used to regulate the continuous and uniform rate of polymer flow. Astatic mixing unit may be placed at the end of the neck tube carryingthe melt from the gear pump into the multilayer feedblock, in order toensure uniform melt stream temperature. The entire melt stream is heatedas uniformly as possible to ensure both uniform flow and minimaldegradation during processing.

[0068] Multilayer feedblocks are designed to divide two or more polymermelt streams into many layers each, interleave these layers, and mergethe many layers of two or more polymers into a single multilayer stream.The layers from any given melt stream are created by sequentiallybleeding off part of the stream from a flow channel into side channeltubes that feed layer slots for the individual layers in the feedblock.Many designs are possible, including those disclosed in U.S. Pat. Nos.3,737,882; 3,884,606; and 3,687,589 to Schrenk et al. Methods have alsobeen described to introduce a layer thickness gradient by controllinglayer flow as described in U.S. Pat. Nos. 3,195,865; 3,182,965;3,051,452; 3,687,589 and 5,094,788 to Schrenk et al, and in U.S. Pat.No. 5,389,324 to Lewis et al. In typical industrial processes, layerflow is generally controlled by choices made in machining the shape andphysical dimensions of the individual side channel tubes and layerslots.

[0069] Applicants have discovered an improved feedblock design thatallows for better control of the layer thickness distribution and of thelayer uniformity. The improved design incorporates modular features sothat only a few sections of the feedblock need to be machined for eachunique film construction, as further described below. The economicadvantage of the modular design is reduction in time, labor, andequipment needed to change from one film construction to another.

[0070]FIG. 3 shows a schematic cross-section feedblock 10, which isenclosed in a housing 12. Within the housing 12 reside an optionalmanifold plate 20 and a gradient plate 30, which in combination, defineat least two supplemental channels, a first channel 22 and a secondchannel 24. As shown, a portion of the bottom surface of manifold plate20 together with a portion of the top surface of gradient plate 30define the supplemental channels 22 and 24. The supplemental channelsare an optional feature of the feedblock, and they help convey resinfrom one position in the feedblock to another position. In addition,plate-type heaters (not shown) can be attached to the external surfacesof the housing 12.

[0071] Residing in gradient plate 30 are at least two flow channels, afirst flow channel 32 and a second flow channel 34. The flow channelsare bounded by a combination of the gradient plate 30 and a feeder tubeplate 40. The first flow channel 32 is in fluid communication with thefirst supplemental channel 22 while second flow channel 34 is in fluidcommunication with second supplemental channel 24. When supplementalchannels are used in combination with flow channels, transfer conduits(not shown) serve as the communication means to connect the two types ofchannels together. Although only a pair of supplemental channels and apair of flow channels are shown, it is within the scope of thisinvention to use more than two channels of each type.

[0072] In the gradient plate 30, each flow channel is machined so thatits cross-section has a central axis of symmetry, such as, e.g., acircle, square, or equilateral triangle. For ease of machining purposes,the square cross-section flow channel is preferably used. Along eachflow channel, the cross-sectional area can remain constant or canchange. The change may be an increase or decrease in area, and adecreasing cross-section is typically referred to as a “taper.” A changein cross-sectional area of the flow channels can be designed to providean appropriate pressure gradient, which affects the layer thicknessdistribution of a multilayer optical film. Thus, the gradient plate canbe changed for different types of multilayer film constructions.

[0073] When the cross-sectional area of the flow channels is made toremain constant, a plot of layer thickness vs. layer number isnon-linear and decreasing. For a given polymer flow, there exists atleast one cross-sectional tapering profile which will result in alinear, decreasing dependency of layer thickness upon layer number,which is sometimes preferred. The taper profile can be found by onereasonably skilled in the art using reliable rheological data for thepolymer in question and polymer flow simulation software known in theart, and should be calculated on a case-by-case basis.

[0074] Referring again to FIG. 3, the feedblock 10 further contains afeeder tube plate 40 that has a first set of conduits 42 and a secondset of conduits 44, each set in fluid communication with flow channels32 and 34 respectively. As used in this document, the “conduits” arealso referred to as “side channel tubes.” Optionally, residing inbetween the two sets of conduits is an axial rod heater 46, used toprovide heat to the resin flowing in the conduits. If desired,temperature can be varied in zones along the length of the axial rodheater. Additional axial rod heaters can be used, for example, oneadjacent to conduit 42 and another adjacent to conduit 44. Each conduitfeeds its own respective slot die 56, which has an expansion section anda slot section. The expansion section typically resides in the feedertube plate 40. If desired, the slot section can reside in a slot plate50. As used in this document, the term “slot die” is synonymous with“layer slot.” The first set of conduits 42 is interleaved with thesecond set of conduits 44 to form alternating layers.

[0075] In use, polymeric resins, in the form of a melt stream, aredelivered to the supplemental channels 22 and 24, if present, from asource, such as an extruder. Typically, a different resin is deliveredto each supplemental channel. For example, resin A is delivered tochannel 22 and resin B is delivered to channel 24 as two distinct meltstreams. If supplemental channels are not used, resin A and resin Bwould be delivered directly to the flow channels 32 and 34. As the meltstream A and melt stream B travel down the flow channels in the gradientplate 30, each melt stream is bled off by the conduits. Because theconduits 42 and 44 are interleaved, they begin the formation ofalternating layers, such as, for example, ABABAB. Each conduit has itsown slot die to begin the formation of an actual layer. The melt streamexiting the slot die contains a plurality of alternating layers. Themelt stream is fed into a compression section (not shown) where thelayers are compressed and also uniformly spread out transversely.Special thick layers known as protective boundary layers (PBLs) may befed nearest to the feedblock walls from any of the melt streams used forthe optical multilayer stack. The PBLs can also be fed by a separatefeed stream after the feedblock. The PBLs function to protect thethinner optical layers from the effects of wall stress and possibleresulting flow instabilities.

[0076] In optical applications, especially for films intended totransmit or reflect a specific color(s), very precise layer thicknessuniformity in the film plane is required. Perfect layer uniformityfollowing a transverse spreading step, occurring in the slot die, isdifficult to achieve in practice. The greater the amount of transversespreading required, the higher the likelihood of non-uniformity in theresulting layer thickness profile. Thus, it is advantageous from thestandpoint of layer thickness profile uniformity (or for film coloruniformity) for the feedblock's slot die to be relatively wide. However,increasing the widths of the slot die results in a larger, heavier, andmore expensive feedblock. It will be apparent that an assessment of theoptimal slot widths must be made individually for each feedblock case,taking into consideration the optical uniformity requirements of theresulting film. This assessment can be done using reliable rheologicaldata for the polymer in question and polymer flow simulation softwareknown in the art, along with a model for feedblock fabrication costs.

[0077] Control of layer thickness is especially useful in producingfilms having specific layer thicknesses or thickness gradient profilesthat are modified in a prescribed way throughout the thickness of themultilayer film. For example, several layer thickness designs have beendescribed for infrared films, which minimize higher order harmonics.Such harmonics can cause color in the visible region of the spectrum.Examples of such film include those described in U.S. Pat. No. RE34,605, incorporated herein by reference, which describes a multilayeroptical interference film comprising three diverse substantiallytransparent polymeric materials, A, B, and C and having a repeating unitof ABCB. The layers have an optical thickness of between about 0.09 and0.45 micrometers, and each of the polymeric materials has a differentindex of refraction, n₁. The film includes polymeric layers of polymersA, B, and C. Each of the polymeric materials has its own differentrefractive index, n_(A), n_(B), n_(C), respectively. A preferredrelationship of the optical thickness ratios of the polymers produces anoptical interference film in which multiple successive higher orderreflections are suppressed. In this embodiment, the optical thicknessratio of first material A, f_(A), is 1/5, the optical thickness ratio ofsecond material B, f_(B), is 1/6, the optical thickness of thirdmaterial C, f_(C) is 1/3, and n_(B)={square root}{square root over(n_(A)n_(C))}.

[0078] For this embodiment, there will be an intense reflection at thefirst order wavelength, while the reflections at the second, third, andfourth order wavelengths will be suppressed. To produce a film thatreflects a broad bandwidth of wavelengths in the solar infrared range(e.g., reflection at from about 0.7 to 2.0 micrometers), a layerthickness gradient may be introduced across the thickness of the film.For example, the layer thicknesses may increase monotonically across thethickness of the film. Preferably, in a three component system of thepresent invention, the first polymeric material (A) differs inrefractive index from the second polymeric material (B) by at leastabout 0.03, the second polymeric material (B) differs in refractiveindex from the third polymeric material (C) by at least about 0.03, andthe refractive index of the second polymeric material (B) isintermediate the respective refractive indices of the first (A) andthird (C) polymeric materials. Polymeric materials can be synthesized tohave the desired index of refraction by using a copolymer or miscibleblend of polymers. For example, the second polymeric material may be acopolymer or miscible blend of the first and third polymeric materials.By varying the relative amounts of monomers in the copolymer or polymersin the blend, any of the first, second, or third materials can beadjusted so that there is a refractive index relationship wheren_(B)={square root}{square root over (n_(A)n_(C))}.

[0079] Another suitable film is described in U.S. Pat. No. 5,360,659,incorporated herein by reference. The patent describes a two componentfilm having six layers alternating repeating unit. The film suppressesthe unwanted second, third, and fourth order reflections in the visiblewavelength region of between about 380-770 nm while reflecting light inthe infrared wavelength region of between about 770-2000 nm. Reflectionshigher than fourth order will generally be in the ultraviolet, notvisible, region of the spectrum or will be of such a low intensity as tobe unobjectionable. The film comprises alternating layers of first (A)and second (B) diverse polymeric materials in which the six layersalternating repeat unit has relative optical thicknesses of about0.778A0.111B0.111A0.778B0.111A0.111B. The use of only six layers in therepeat unit results in more efficient use of material and simplermanufacture than previous designs. A repeat unit gradient may beintroduced across the thickness of the film. Thus, in one embodiment,the repeat unit thicknesses will increase linearly across the thicknessof the film. By linearly, it is meant that the repeat unit thicknessesincrease at a constant rate across the thickness of the film. In someembodiments, it may be desirable to force the repeat unit opticalthickness to double from one surface of the film to another. The ratioof repeat unit optical thicknesses can be greater or less than two aslong as the short wavelength range of the reflectance band is above 770nm and the long wavelength edge is about 2000 nm. Other repeat unitgradients may be introduced by using logarithmic and/or quarticfunctions. A logarithmic distribution of repeat unit thicknesses willprovide nearly constant reflectance across the infrared band.

[0080] In an another embodiment, the two component film may comprise afirst portion and a second portion of alternating layers. The firstportion has the six layers alternating layer repeat unit that reflectsinfrared light of wave lengths between about 1200-2000 nm. The secondportion of alternating layers has an AB repeat unit, has substantiallyequal optical thickness, and reflects infrared light of wavelengthsbetween about 770-1200 nm. Such a combination of alternating layersresults in reflection of light across the infrared wavelength regionthrough about 2000 nm. The combination is commonly known as a “hybriddesign.” Preferably, the first portion of the alternating layers has arepeat unit gradient of about 5/3:1, and the second portion ofalternating layers have a layer thickness gradient of about 1.5:1. Thehybrid design may be provided as described for example in U.S. Pat. No.5,360,659, but has broader application in that it is useful with any ofthe broadband infrared reflectors or multicomponent optical designsdescribed herein.

[0081] Another useful film design is described in U.S. patentapplication Ser. No. 09/006,118 filed Jan. 13, 1998 entitled“Multicomponent Reflective Film,” which is incorporated herein byreference. Optical films and other optical bodies are described whichexhibit a first order reflection band for at least one polarization ofelectromagnetic radiation in a first region of the spectrum. Suchoptical films suppress at least the second, and preferably also at leastthe third, higher order harmonics of the first reflection band, whilethe percentage reflection of the first order harmonic remainsessentially constant, or increases, as a function of angle of incidence.This is accomplished by forming at least a portion of the optical bodyout of polymeric materials A, B, and C which are arranged in a repeatingsequence ABC, wherein A has refractive indices n_(x) ^(A), n_(y) ^(A),and n_(z) ^(A) along mutually orthogonal axes x, y, and z, respectively.Similarly, material B has refractive indices n_(x) ^(B), n_(y) ^(B), andn_(z) ^(B) along axes x, y and z, respectively, and C has refractiveindices n_(x) ^(C), n_(y) ^(C) and n_(z) ^(C) along axes x, y, and z,respectively. The z-axis is orthogonal to the plane of the film oroptical body. In the optical film, n_(x) ^(A)>n_(x) ^(B)>n_(x) ^(C) orn_(y) ^(A)>n_(y) ^(B)>n_(y) ^(C), and n_(z) ^(C)≧n_(z) ^(B)≧n_(z) ^(A).Preferably, at least one of the differences n_(z) ^(A)-n_(z) ^(B) andn_(z) ^(B)-n_(z) ^(C) is less than about −0.05.

[0082] By designing the film or optical body within these constraints,at least some combination of second, third and fourth higher-orderreflections can be suppressed without a substantial decrease of thefirst harmonic reflection with angle of incidence, particularly when thefirst reflection band is in the infrared region of the spectrum. Suchfilms and optical bodies are particularly useful as IR mirrors, and maybe used advantageously as window films and in similar applications whereIR protection is desired but good transparency and low color areimportant.

[0083] A modular feedblock of the type described herein, having achangeable gradient plate adaptable to vary the thickness of individuallayer thicknesses or layer thickness profiles without necessitatingchanging or re-machining the entire feedblock assembly, is especiallyuseful for modifying layer thickness profiles as described above.

[0084] The various layers in the film preferably have differentthicknesses across the film. This is commonly referred to as the layerthickness gradient. A layer thickness gradient is selected to achievethe desired band width of reflection. One common layer thicknessgradient is a linear one, in which the thickness of the thickest layerpairs is a certain percent thicker than the thickness of the thinnestlayer pairs. For example, a 1.055:1 layer thickness gradient means thatthe thickest layer pair (adjacent to one major surface) is 5.5% thickerthan the thinnest layer pair (adjacent to the opposite surface of thefilm). In another embodiment, the layer thickness could decrease,increase, and decrease again from one major surface of the film to theother. This is believed to provide sharper bandedges, and thus a sharperor more abrupt transition from reflective to transmissive regions of thespectrum. This preferred method for achieving sharpened bandedges isdescribed more fully in U.S. patent application Ser. No. 09/006,085entitled “Optical Film with Sharpened Bandedge” filed Jan. 13, 1998, thecontents of which are herein incorporated by reference.

[0085] The method of achieving sharpened band edges will be brieflydescribed for a multilayer film having layers arranged in an alternatingsequence of two optical materials, “A” and “B”. Three or more distinctoptical materials can be used in other embodiments. Each pair ofadjacent “A” and “B” layers make up an optical repeating unit (ORU),beginning at the top of the film with ORU1 and ending with ORU6, withthe ORUs having optical thicknesses OT₁, OT₂, . . . OT₆. For maximumfirst order reflectance (M=1 in equation I) at a design wavelength, eachof the ORUs should have a 50% f-ratio with respect to either the A or Blayer. The A layers can be considered to have a higher X—(in-plane)refractive index than the B layers because the former are shown to bethinner than the latter. ORUs 1-3 may be grouped into a multilayer stackS1 in which the optical thickness of the ORUs decrease monotonically inthe minus-Z direction, while ORUs 4-6 may be grouped into anothermultilayer stack S2 in which the optical thickness of the ORUs increasemonotonically. Such thickness profiles are helpful in producingsharpened spectral transitions. In contrast, thickness profiles ofpreviously known films typically increase or decrease monotonically inonly one direction. If desired for some applications, a discontinuity inoptical thickness can be incorporated between the two stacks to giverise to a simple notch transmission band spectrum.

[0086] Other thickness gradients may be designed which improve peaktransmission and to make even steeper band edges (narrower transmissionband). This can be achieved by arranging the individual layers intocomponent multilayer stacks where one portion of the stacks hasoppositely curved thickness profiles and the adjacent portions of thestacks have a slightly curved profile to match the curvature of thefirst portion of the stacks. The curved profile can follow any number offunctional forms. The main purpose of the form is to break the exactrepetition of thickness present in a quarter wave stack with layerstuned to only a single wavelength. The particular function used is anadditive function of a linear profile and a sinusoidal function to curvethe profile with an appropriate negative or positive first derivative.An important feature is that the second derivative of the ORU thicknessprofile be positive for the red (long wavelength) band edge of areflectance stack and negative for the blue (short wavelength) band edgeof a reflectance stack. The opposite sense is required if one refers tothe band edges of the notched transmission band. Other embodimentsincorporating the same principle include layer profiles that havemultiple points with a zero value of the first derivative. In all caseshere, the derivatives refer to those of a best fit curve fitted throughthe actual ORU optical thickness profile, which can contain smallstatistical errors of less than 10% sigma, one standard deviation inoptical thickness values.

[0087] The multilayer stack exiting the feedblock may then directlyenter a final shaping unit such as a die. Alternatively, the stream maybe split, preferably normal to the layers, to form two or moremultilayer streams that may be recombined by stacking. The stream mayalso be split at an angle other than that normal to the layers. A flowchanneling system that splits and stacks the streams is called amultiplier or interfacial surface generator (ISG). The width of thesplit streams can be equal or unequal. The multiplier ratio is definedby the ratio of the wider to narrower stream widths. Unequal streamswidths (i.e., multiplier ratios greater than unity) can be useful increating layer thickness gradients. In the case of unequal streams, themultiplier should spread the narrower stream and/or compress the widerstream transversely to the thickness and flow directions to ensurematching layer widths upon stacking. Many designs are possible,including those disclosed in U.S. Pat. Nos. 3,565,985; 3,759,647;5,094,788; and 5,094,793 to Schrenk et al. In typical practice, the feedto a multiplier is rectangular in cross-section, the two or more splitstreams are also rectangular in cross-section, and rectangularcross-sections are retained through the flow channels used to re-stackthe split streams. Preferably, constant cross-sectional area ismaintained along each split stream channel, though this is not required.

[0088] Each original portion of the multilayer stack that exits thefeedblock manifold, excluding PBLs, is known as a packet. In a film foroptical applications, each packet is designed to reflect, transmit, orpolarize over a given band of wavelengths. More than one packet may bepresent as the multilayer stack leaves the feedblock. Thus, the film maybe designed to provide optical performance over dual or multiple bands.These bands may be separate and distinct, or may be overlapping.Multiple packets may be made of the same or of different combinations oftwo or more polymers. Multiple packets in which each packet is made ofthe same two or more polymers may be made by constructing the feedblockand its gradient plate in such a way that one melt train for eachpolymer feeds all packets, or each packet may be fed by a separate setof melt trains. Packets designed to confer on the film other non-opticalproperties, such as physical properties, may also be combined withoptical packets in a single multilayer feedblock stack.

[0089] An alternative to creating dual or multiple packets in thefeedblock is to create them from one feedblock packet via the use of amultiplier with multiplier ratio greater than unity. Depending on thebandwidth of the original packet and the multiplier ratio, the resultingpackets can be made to overlap in bandwidth or to leave between them abandwidth gap. It will be evident to one skilled in the art that thebest combination of feedblock and multiplier strategies for any givenoptical film will depend on many factors, and must be determined on anindividual basis.

[0090] Prior to multiplication, additional layers can be added to themultilayer stack. These outer layers perform as PBLs, but this time,within the multiplier. After multiplication and stacking, part of thePBL streams will form internal boundary layers between optical layers,while the rest will form skin layers. Thus the packets are separated byPBLs in this case. Additional PBLs can be added and additionalmultiplication steps may be accomplished prior to final feed into aforming unit such as a die. Prior to the final feed, additional layerscan be added to the outside of the multilayer stack, whether or notmultiplication has been performed, and whether or not PBLs have beenadded prior to the multiplication step. The additional layers form thefinal skin layers and the external portions of the earlier-applied PBLswill form sub-skins under these final skin layers. The die performs theadditional compression and width spreading of the melt stream. Again,the die (including its internal manifold, pressure zones, etc.) isdesigned to create uniformity of the layer distribution across the webas it exits the die.

[0091] Skin layers are frequently added to the multilayer stack toprotect the thinner optical layers from the effects of wall stress andpossible resulting flow instabilities. Other reasons for adding a thicklayer at the surface(s) of the film include, e.g., surface propertiessuch as adhesion, coatability, release, coefficient of friction, andbarrier properties, weatherability, scratch and abrasion resistance, andothers. In multilayer films that are subsequently uniaxially or veryunequally biaxially drawn, “splittiness,” (i.e., the tendency to tear orfail easily along the more highly drawn direction), can be substantiallysuppressed by choosing a skin layer polymer that (1) adheres well to thesub-skin or nearest optical layer polymer and (2) is less prone toorientation upon draw. An example of a useful skin layer, where theoptical stack contains a PEN homopolyer, is a copolymer of PEN having acomonomer content sufficient to suppress crystallinity and/orcrystalline orientation. Marked suppression of splittiness is observedin such a structure, compared to a similar film without the coPEN skinlayer(s), when the films are highly drawn in one planar direction andundrawn or only slightly drawn in the orthogonal planar direction. Oneskilled in the art will be able to select similar skin layer polymers tocomplement other optical layer polymers and/or sub-skin polymers.

[0092] Temperature control is important in the feedblock and subsequentflow leading to casting at the die lip. While temperature uniformity isoften desired, in some cases, deliberate temperature gradients in thefeedblock or temperature differences of up to about 40° C. in the feedstreams can be used to narrow or widen the stack layer thicknessdistribution. Feed streams into the PBL or skin blocks can also be setat different temperatures than the feedblock average temperature. Often,the PBL or skin streams are about 40° C. higher than the feed streamtemperature to reduce viscosity or elasticity in the protective streamsand thus enhance their effectiveness as protective layers. Sometimes,the protective streams' temperature can be decreased up to about 40° C.to improve the rheology matching between them and the rest of the flowstream. For example, decreasing the temperature of a low viscosity skinmay enhance viscosity matching and enhance flow stability. Other times,elastic effects need to be matched.

[0093] Conventional means for heating the feedblock-multiplier-dieassembly, namely, the use of insertion- or rod- or cartridge-typeheaters fitted into bores in the assembly, are frequently incapable ofproviding the temperature control required for the inventive opticalfilms. Preferably, heat is provided uniformly from outside the assemblyby (i) tiling its exterior with plate-type heaters, (ii) insulatingthoroughly the entire assembly, or (iii) combining the two techniques.Plate-type heaters typically use a resistance-heating element embeddedin a metal material, such as cast aluminum. Such heaters can distributeheat uniformly to an apparatus, such as, e.g., the feedblock.

[0094] The use of insulation to control heat flow is not new. It is,however, typically not done in film extrusion due to the possibility ofpolymer melt leakage from the assembly onto the insulation. Because ofthe need to regulate layer flows very precisely, such leakage cannot betolerated in the feedblock-multiplier-die assemblies used for theinventive optical films. Thus, feedblocks, multipliers, and dies arecarefully designed, machined, assembled, connected, and maintained so asto prevent polymer melt leakage, and insulation of the assembly becomesboth feasible and preferred.

[0095] An insertion- or rod- or cartridge-type heater, having both aspecific design and specific placement within the feedblock, isadvantageous both for maintaining constant temperature in the feedblockand for creating a temperature gradient of up to about 40° C. Thisheater, called an axial rod heater, consists of a heater placed in abore through the feedblock and oriented in a direction normal to thelayer plane, preferably very near an imaginary line through the pointswhere each side channel tube feeds a slot die. More preferably, in thecase of coextrusion of a first polymer and a second polymer, the borefor the axial rod heater will be located both near an imaginary linethrough the points where each side channel tube feeds a slot die, andalso equidistant from the side channel tubes carrying the first polymerand the side channel tubes carrying the second polymer. Further, theaxial rod heater is preferably of a type that can provide a temperaturegradient or a multiplicity of discrete temperatures along its length,either by variation in electrical resistance along its length, or bymulti-zone control, or by other means known in the art. Such a heater,used in conjunction with the plate-type heaters described above, theinsulation described above, or both, provides superior temperaturecontrol and/or uniformity to traditional means. Such superior controlover layer thickness and gradient layer thickness distribution isespecially important in controlling the positions and profiles ofreflection bands as described in U.S. patent applications Ser. No.09/006,085 entitled “Optical Film with Sharpened Bandedge” and Ser. No.09/006,591 entitled “Color Shifting Film,” both filed Jan. 13, 1998 andthe contents of which are incorporated herein by reference.

[0096] Shear rate is observed to affect viscosity and other rheologicalproperties, such as elasticity. Flow stability sometimes appears toimprove by matching the relative shape of the viscosity (or otherrheological function) versus shear rate curves of the coextrudedpolymers. In other words, minimization of maximal mismatch between suchcurves may be an appropriate objective for flow stability. Thus,temperature differences at various stages in the flow can help tobalance shear or other flow rate differences over the course of thatflow.

[0097] The web is cast onto casting roll, sometimes referred to as acasting wheel or casting drum. The casting roll is preferably chilled toquench the web and begin the formation of a multilayer cast film.Preferably, casting is assisted by electrostatic pinning, the details ofwhich are well-known in the art of polyester film manufacture. For theinventive optical films, care should be exercised in setting theparameters of the electrostatic pinning apparatus. Periodic cast webthickness variations along the extrusion direction of the film,frequently referred to as “pinning chatter,” should be avoided to theextent possible. Adjustments to the current, voltage, pinning wirethickness, and pinning wire location with respect to the die and thecasting chill roll are all known to have an affect, and should be set ona case-by case basis by one skilled in the art.

[0098] The web can sometimes attain a sidedness in surface texture,degree of crystallinity, or other properties due to wheel contact on oneside and merely air contact on the other. This can be desirable in someapplications and undesirable in others. When minimization of suchsidedness differences is desired, a nip roll can be used in combinationwith the casting roll to enhance quenching or to provide smoothing ontowhat would otherwise be the air side of the cast web.

[0099] In some cases, it is important that one side of the multilayerstack be the side chosen for the superior quench that is attained on thechill roll side. For example, if the multilayer stack consists of adistribution of layer thicknesses, it is frequently desired to place thethinnest layers nearest the chill roll. This is discussed in detail inU.S. patent application Ser. No. 08/904,325, entitled “Method for MakingOptical Films Having Thin Optical Layers,” which is incorporated hereinby reference.

[0100] In some cases, it is desired to provide the film with a surfaceroughness or surface texture to improve handling in winding and/orsubsequent conversion and use. A specific example germane to theinventive optical films arises when they are intended for use inintimate contact with a glass plate or a second film. In such cases,selective “wetting out” of the optical film onto the plate or secondfilm can result in the phenomenon known as “Newton's Rings,” whichdamages the uniformity of the optics over large surface areas. Atextured or rough surface prevents the intimacy of contact required forwetting out thereby minimizing or eliminating the appearance of Newton'sRings.

[0101] It is well known in the polyester film art to include smallamounts of fine particulate materials, often referred to as “slipagents,” to provide such surface roughness or texture. The use of slipagents can be incorporated into the inventive optical films. However,the inclusion of slip agent particulates can introduce a small amount ofhaze and can decrease the optical transmission of the film. Inaccordance with the present invention, Newton's Rings can be effectivelyprevented, without the use of slip agents, if surface roughness ortexture is provided by contacting the cast web with a micro-embossingroll during film casting. Preferably, the micro-embossing roll willserve as a nip roll to the casting wheel. Alternatively, the castingwheel itself may be micro-textured to provide a similar effect. Further,both a micro-embossing casting wheel and a micro-embossing nip roll maybe used together to provide a film that is micro-embossed on both sides.

[0102] Further, Applicants found that the use of a smooth nip roll atthe casting roll, in addition to aiding quench at what would otherwisebe the air side of the film, as already discussed above, can alsosignificantly reduce the magnitude of die lines, pinning chatter, andother thickness fluctuations. The web may be cast to a uniform thicknessacross the web or a deliberate profiling of the web thickness may beinduced using die lip controls. Such profiles may improve uniformity bythe end of the film process. In other cases, a uniform cast thicknessprovides best uniformity at the end of the film process. Controllingvibrations in the process equipment is also important to reduce“chatter” in the cast multilayer web.

[0103] Residence times in the various process stages may also beimportant even at a fixed shear rate. For example, interdiffusionbetween layers can be altered and controlled by adjusting residencetimes. “Interdiffusion,” as used in this document, refers to minglingand reactive processes between materials of the individual layersincluding, for example, various molecular motions such as normaldiffusion, cross-linking reactions, or transesterification reactions.Sufficient interdiffusion is desirable to ensure good interlayeradhesion and prevent delamination. However, too much interdiffusion canlead to deleterious effects, such as the substantial loss ofcompositional distinctness between layers. Interdiffusion can alsoresult in copolymerization or mixing between layers, which may reducethe ability of a layer to be oriented when drawn. The scale of residencetime on which such deleterious interdiffusion occurs is often muchlarger (e.g., by an order of magnitude) than that required to achievegood interlayer adhesion, thus the residence time can be optimized.However, some large-scale interdiffusion may be useful in profiling theinterlayer compositions, for example to make rugate structures.

[0104] The effects of interdiffusion can also be altered by furtherlayer compression. Thus, the effect at a given residence time is also afunction of the state of layer compression during that interval relativeto the final layer compression ratio. As thinner layers are moresusceptible to interdiffusion, they are typically placed closest to thecasting wheel for maximal quenching.

[0105] Applicants also found that interdiffusion can be enhanced afterthe multilayer film has been cast, quenched, and drawn, via heat settingat an elevated temperature. Heat setting is normally done in the tenteroven in a zone subsequent to the transverse drawing zone. Normally, forpolyester films, the heat setting temperature is chosen to maximizecrystallization rate and optimize dimensional stability properties. Thistemperature is normally chosen to be between the glass transition andmelting temperatures, and not very near either temperature. Selection ofa heat set temperature closer to the melting point of the lowest-meltingpolymer among those polymers in the multilayer film which are desired tomaintain orientation in the final state results in a marked improvementin interlayer adhesion. This is unexpected due to the short residencetimes involved in heat setting on line, and the non-molten nature of thepolymers at this process stage. Further, while off-line heat treatmentsof much longer duration are known to improve interlayer adhesion inmultilayer films, these treatments also tend to degrade otherproperties, such as modulus or film flatness, which was not observedwith on-line elevated-temperature heat setting treatments.

[0106] Conditions at the casting wheel are set according to the desiredresult. Quenching temperatures must be cold enough to limit haze whenoptical clarity is desired. For polyesters, typical casting temperaturesrange between 10° C. and 60° C. The higher portion of the range may beused in conjunction with smoothing or embossing rolls while the lowerportion leads to more effective quenching of thick webs. The speed ofthe casting wheel may also be used to control quench and layerthickness. For example, the extruder pumping rates may be slowed toreduce shear rates or increase interdiffusion while the casting wheel isincreased in speed to maintain the desired cast web thickness. The castweb thickness is chosen so that the final layer thickness distributioncovers the desired spectral band at the end of all drawing withconcomitant thickness reductions.

[0107] The multilayer web is drawn to produce the final multilayeroptical film. A principal reason for drawing is to increase the opticalpower of the final optical stack by inducing birefringence in one ormore of the material layers. Typically, at least one material becomesbirefringent under draw. This birefringence results from the molecularorientation of the material under the chosen draw process. Often thisbirefringence greatly increases with the nucleation and growth ofcrystals induced by the stress or strain of the draw process (e.g.stress-induced crystallization). Crystallinity suppresses the molecularrelaxation, which would inhibit the development of birefringence, andcrystals may themselves also orient with the draw. Sometimes, some orall of the crystals may be pre-existing or induced by casting orpreheating prior to draw. Other reasons to draw the optical film mayinclude, but are not limited to, increasing throughput and improving themechanical properties in the film.

[0108] In one typical method for making a multilayer optical polarizer,a single drawing step is used. This process may be performed in a tenteror a length orienter. Typical tenters draw transversely (TD) to the webpath, although certain tenters are equipped with mechanisms to draw orrelax (shrink) the film dimensionally in the web path or machinedirection (MD). Thus, in this typical method, a film is drawn in onein-plane direction. The second in-plane dimension is either heldconstant as in a conventional tenter, or is allowed to neck into asmaller width as in a length orienter. Such necking in may besubstantial and increases with draw ratio. For an elastic,incompressible web, the final width may be estimated theoretically asthe reciprocal of the square root of the lengthwise draw ratio times theinitial width. In this theoretical case, the thickness also decreases bythis same proportion. In practice, such necking may produce somewhatwider than theoretical widths, in which case the thickness of the webmay decrease to maintain approximate volume conservation. However,because volume is not necessarily conserved, deviations from thisdescription are possible.

[0109] In one typical method for making a multilayer mirror, a two stepdrawing process is used to orient the birefringent material in bothin-plane directions. The draw processes may be any combination of thesingle step processes described that allow drawing in two in-planedirections. In addition, a tenter that allows drawing along MD, e.g. abiaxial tenter, which can draw in two directions sequentially orsimultaneously, may be used. In this latter case, a single biaxial drawprocess may be used.

[0110] In still another method for making a multilayer polarizer, amultiple drawing process is used that exploits the different behavior ofthe various materials to the individual drawing steps to make thedifferent layers comprising the different materials within a singlecoextruded multilayer film possess different degrees and types oforientation relative to each other. Mirrors can also be formed in thismanner. Such optical films and processes are described further in U.S.patent application Ser. No. 09/006,455 filed Jan. 13, 1998 entitled “AnOptical Film and Process for Manufacture Thereof,” the contents of whichis incorporated by reference.

[0111] Drawing conditions for multilayer optical polarizer films areoften chosen so that a first material becomes highly birefringentin-plane after draw. A birefringent material may be used as the secondmaterial. If the second material has the same sense of birefringence asthe first (e.g. both materials are positively birefringent), then it isusually preferred to choose the second material so that is remainsessentially isotropic. In other embodiments, the second material ischosen with a birefringence opposite in sense to the first material whendrawn (e.g., if the first material is positively birefringent, thesecond material is negatively birefringent). For a positivelybirefringent first material, the direction of highest in-planerefractive index, the first in-plane direction, coincides with the drawdirection, while the direction of lowest in-plane refractive index forthe first material, the second in-plane direction, is perpendicular tothe first direction. Similarly, for multilayer mirror films, a firstmaterial is chosen to have large out-of-plane birefringence, so that thein-plane refractive indices are both higher than the initial isotropicvalue in the case of a positively birefringent material (or lower in thecase of a negatively birefringent material). In the mirror case, it isoften preferred that the in-plane birefringence is small so that thereflections are similar for both polarization states, i.e. a balancedmirror. The second material for the mirror case is then chosen to beisotropic, or birefringent in the opposite sense, in similar fashion tothe polarizer case.

[0112] In another embodiment of multilayer optical films, polarizers maybe made via a biaxial process. In still another embodiment, balancedmirrors may be made by a process that creates two or more materials ofsignificant in-plane birefringence and thus in-plane asymmetry such thatthe asymmetries match to form a balanced result, e.g. nearly equalrefractive index differences in both principal in-plane directions.

[0113] In certain processes, rotation of these axes can occur due to theeffects of process conditions including tension changes down web. Thisis sometimes referred to as “bow-forward” or “bow-back” in film made onconventional tenters. Uniform directionality of the optical axes isusually desirable for enhanced yield and performance. Processes thatlimit such bowing and rotation, such as tension control or isolation viamechanical or thermal methods, can be used.

[0114] Frequently, it is observed that drawing film transverse to themachine direction in a tenter is non-uniform, with thickness,orientation, or both changing as the film approaches the gripped edgesof the web. Typically, these changes are consistent with the assumptionof a cooler web temperature near the gripped edges than in the webcenter. The result of such non-uniformity can be a serious reduction inusable width of the finished film. This restriction can be even moresevere for the optical films of the present invention, as very smalldifferences in film thickness can result in non-uniformity of opticalproperties across the web. Drawing, thickness, and color uniformity, asApplicants recognize, can be improved by the use of infrared heaters toheat further the edges of the film web near the tenter grippers. Suchinfrared heaters can be used before the tenter's preheat zone, in thepreheat zone, in the stretch zone, or in a combination of locations. Oneskilled in the art will appreciate the many options for zoning andcontrolling the addition of infrared heat. Further, the possibilitiesfor combining infrared edge heating with changes in the cast web'scross-web thickness profile will also be apparent.

[0115] For certain of the inventive multilayer optical films, it isdesirable to draw the film in such a way that one or more properties,measured on the finished films, have identical values in the machine andtransverse directions. Such films are often referred to as “balanced”films. Machine- and transverse-direction balance can be achieved byselecting process conditions using techniques well known in the art ofbiaxially oriented film making. Typically, process parameters exploredinclude machine-direction orientation preheat temperature, stretchtemperature, and draw ratio, tenter preheat temperature, tenter stretchtemperature, and tenter draw ratio, and, sometimes, parameters relatedto the post-stretching zones of the tenter. Other parameters may also besignificant. Typically, designed experiments are performed and analyzedto arrive at appropriate combinations of conditions. Those skilled inthe art will appreciate the need to perform such an assessmentindividually for each film construction and each film line on which itis to be made.

[0116] Similarly, parameters of dimensional stability (such as shrinkageat elevated temperature and reversible coefficient of thermal expansion)are affected by a variety of process conditions. Such parametersinclude, but are not limited to, heat set temperature, heat setduration, transverse direction dimensional relaxation (“toe-in”) duringheat set, web cooling, web tension, and heat “soaking” (or annealing)after winding into rolls. Again, designed experiments can be performedby one skilled in the art to determine optimum conditions for a givenset of dimensional stability requirements, for a given film composition,and for a given film line.

[0117] In general, multilayer flow stability is achieved by matching orbalancing the rheological properties, such as viscosity and elasticity,between the first and second materials to within a certain tolerance.The level of required tolerance or balance also depends on the materialsselected for the PBL and skin layers. In many cases, it is desirable touse one or more of the optical stack materials individually in thevarious PBL or skin layers. For polyesters, the typical ratio betweenhigh and low viscosity materials is no more than 4:1, preferably no morethan 2:1, and most preferably no more than 1.5:1 for the processconditions typical of feedblocks, multipliers, and dies. Using the lowerviscosity optical stack material in the PBL and skin layers usuallyenhances flow stability. More latitude in the requirements for a secondmaterial to be used with a given first material is often gained bychoosing additional materials for the PBL and skin layers. Often, theviscosity requirements of these third materials (PBL and skin layers)are then balanced with the effective average viscosities of themultilayer stack comprising the first and second materials. Typically,the viscosity of the PBL and skin layers should be lower than this stackaverage for maximal stability. If the process window of stability islarge, higher viscosity materials can be used in these additionallayers, for example, to prevent sticking to rollers downstream ofcasting in a length orienter.

[0118] Draw compatibility means that the second material can undergo thedraw processing needed to achieve the desired birefringence in the firstmaterial without causing deleterious effects to the multilayer film,such as breakage, voiding, or stress whitening. These effects can causeundesired optical properties. Draw compatibility usually requires thatthe glass transition temperature of the second material be no more thanabout 40° C. higher than that of the first material. This limitation canbe ameliorated (1) by very fast drawing rates that make the orientationprocess for the first material effective even at higher temperatures or(2) by crystallization or cross-linking phenomena that also enhance theorientation of the first material at such higher temperatures. Also,draw compatibility requires that the second material can achieve thedesired optical state at the end of processing, whether this is anessentially isotropic state or a highly birefringent state.

[0119] In the case of a second material that is to remain isotropicafter final processing, at least three methods of material selection andprocessing can be used to meet this second requirement for drawcompatibility. First, the second material can be inherentlynon-birefringent. An example of an inherently non-birefringent materialis poly methylmethacrylate because it remains optically isotropic (asmeasured by refractive index) even if there is substantial molecularorientation after drawing. Second, the second material can be chosen soas to remain unoriented at the draw conditions of the first material,even though it could be made birefringent if drawn under differentconditions. Third, the second material can orient during the drawprocess provided it may lose the orientation so gained in a subsequentprocess, such as a heat-setting step. In the case of multiple drawingschemes in which the final desired film contains more than one highlybirefringent material (e.g. a polarizer made in certain biaxial drawingschemes), draw compatibility may not require any of these methods.Alternatively, the third method may be applied to achieve isotropy aftera given drawing step, or any of these methods may be used for third orfurther materials.

[0120] Draw conditions can also be chosen to take advantage of thedifferent visco-elastic characteristics of the first and second opticalmaterials, as well as any materials used in the skin and PBL layers,such that the first material becomes highly oriented during draw whilethe second remains unoriented or only slightly oriented after drawaccording to the second scheme described above. Visco-elasticity is afundamental characteristic of polymers. The visco-elasticitycharacteristics of a polymer may be used to describe its tendency toreact to strain like a viscous liquid or an elastic solid. At hightemperatures and/or low strain rates, polymers tend to flow when drawnlike a viscous liquid with little or no molecular orientation. At lowtemperatures and/or high strain rates, polymers tend to draw elasticallylike solids with concomitant molecular orientation. A low temperatureprocess is typically considered take place near the polymeric material'sglass transition temperature, while a high temperature process takesplace substantially above the glass temperature.

[0121] Visco-elastic behavior is generally the result of the rate ofmolecular relaxation in a polymeric material. In general, molecularrelaxation is the result of numerous molecular mechanisms, many of whichare molecular weight dependent. Thus, polydisperse polymeric materialshave a distribution of relaxation times, with each molecular weightfraction in the polydisperse polymer having its own longest relaxationtime. The rate of molecular relaxation can be characterized by anaverage longest overall relaxation time (i.e., overall molecularrearrangement) or a distribution of such times. The precise numericalvalue for the average longest relaxation time for a given distributionis a function of how the various times in the distribution are weightedin the average. The average longest relaxation time typically increaseswith decreasing temperature and becomes very large near the glasstransition temperature. The average longest relaxation time can also beincreased by crystallization and/or crosslinking in the polymericmaterial which, for practical purposes, inhibits any relaxation underprocess times and temperatures typically used. Molecular weight anddistribution, as well as chemical composition and structure (e.g.,branching), can also effect the longest relaxation time.

[0122] The choice of resin strongly effects the characteristicrelaxation time. Average molecular weight, MW, is a particularlysignificant factor. For a given composition, the characteristic timetends to increase as a function of molecular weight (typically as the 3to 3.5 power of molecular weight) for polymers whose molecular weight iswell above the entanglement threshold. For unentangled polymers, thecharacteristic time tends to increase as a weaker function of molecularweight. Because polymers below this threshold tend to be brittle whenbelow their glass transition temperatures and are usually undesirable,they are not the principal focus here. However, certain lower molecularmaterials may be used in combination with layers of higher molecularweight as could low molecular weight rubbery materials above the glasstransition, e.g. an elastomeric or tacky layer. Inherent or intrinsicviscosity, IV, rather than average molecular weight, is usually measuredin practice. The IV varies as MWα where α is the solvent dependentMark-Houwink exponent. The exponent α increases with solubility of thepolymer. Typical values of α might be 0.62 for PEN (polyethylenenaphthalate) and 0.68 for PET (polyethylene terephthalate), bothmeasured in solutions of 60:40 Phenol:ortho-Dichlorobenzene, withintermediate values for a copolymer of the two (e.g., coPEN). PBT(polybutylene terephthalate) would be expected to have a still largervalue of α than PET, as would polyesters of longer alkane glycols (e.g.hexane diol) assuming improved solubility in the chosen solvent. For agiven polymer, better solvents would have higher exponents than thosequoted here. Thus, the characteristic time is expected to vary as apower law with IV, with its power exponent between 3/α and 3.5/α. Forexample, a 20% increase in IV of a PEN resin is expected to increase theeffective characteristic time. Thus the Weissenberg Number (as definedbelow) and the effective strength of the drawing flow, at a givenprocess temperature and strain rate by a factor of approximately 2.4 to2.8. Since a lower IV resin will experience a weaker flow, relativelylower IV resins are preferred in the present invention for the case of asecond polymer of desired low final birefringence, and higher IV resinsare preferable for the stronger flows required of the first polymer ofhigh birefringence. The limits of practice are determined by brittlenesson the low IV end and by the need to have adequate rheologicalcompatibility during the coextrusion. In other embodiments, in whichstrong flows and high birefringence are desired in both a first andsecond material, higher IV may be desired for both materials. Otherprocessing considerations, such as upstream pressure drops as might befound in the melt stream filters, can also become important.

[0123] The severity of a strain rate profile can be characterized in afirst approximation by a Weissenberg number (Ws) which is the product ofthe strain rate and the average longest relaxation time for a givenmaterial. The threshold Ws value between weak and strong draw (belowwhich, and above which, the material remains isotropic or experiencesstrong orientation, crystallization and high birefringence,respectively) depends on the exact definition of this average longestrelaxation time as an average of the longest relaxation times in thepolydisperse polymeric material. It will be appreciated that theresponse of a given material can be altered by controlling the drawingtemperature, rate and ratio of the process. A process which occurs in ashort enough time and/or at a cold enough temperature to inducesubstantial molecular orientation is an orienting or strong drawprocess. A process which occurs over a long enough period and/or at hotenough temperatures such that little or no molecular orientation occursis a non-orienting or weak process.

[0124] Another critical issue is the duration of the draw process.Strong draw processes typically need enough duration (that is, a highenough draw ratio) to accomplish sufficient orientation, e.g. to exceedthe threshold for strain-induced crystallization, thereby achieving highbirefringence in the first material. Thus, the strain rate historyprofile, which is the collection of the instantaneous strain rates overthe course of the drawing sequence, is a key element of the drawprocess. The accumulation of the instantaneous strain rates over theentire draw process determines the final draw ratio. The temperature andstrain rate draw profile history determine the draw ratio at which thefirst polymer experiences the onset of strain-induced crystallization,given the characteristic time and supercooling of that polymer.Typically, the onset draw ratio decreases with increasing Ws. For PET,experimental evidence suggests this onset draw ratio has a limit between1.5 and 2 at very high rates of strain. At lower rates of strain, theonset draw ratio for PET can be over 3. The final level of orientationoften correlates with the ratio of the final draw ratio to the onsetdraw ratio.

[0125] Temperature has a major effect on the characteristic averagelongest relaxation time of the material, and is thus a major factor indetermining whether a given material experiences a weak or strong flow.The dependence of the characteristic time on temperature can bequantified by the well known WLF equation [See. J. D. Ferry,Viscoelastic Properties of Polymers, John Wiley & Sons, New York, 1970].This equation contains three parameters, c₁, c₂ and T₀. Often, T₀ isassociated with the glass transition temperature, T_(g). Using theapproximate “universal” values for c₁ and c₂, applicable as a firstestimate for many polymers, the WLF equation shows the large dependenceon relaxation times with temperature. For example, using a relaxationtime at 5° C. higher than the T_(g) as a value for comparison, therelaxation times at 10° C., 15° C., and 20° C. higher than T_(g) areapproximately 20, 250 and 2000 times shorter, respectively. Greateraccuracy for WLF parameters can be obtained by using empirical curvefitting techniques for a particular class of polymers, e.g. polyesters.Thus, to a first approximation, the single most important parameter fortemperature effects on the characteristic time is T_(g). The larger thetemperature difference between the web temperature and T_(g), thesmaller the characteristic time and thus the weaker the draw flow.Further, it is reiterated that this discussion is most pertinent to thedraw process prior to crystallization, especially strain inducedcrystallization. After crystallization occurs, the presence of crystalscan further retard relaxation times and convert otherwise weak flows tostrong flows.

[0126] By selecting the materials and process conditions inconsideration of the orienting/non-orienting response of the materials,a film can be constructed such that the first material is oriented andbirefringent and the second material is essentially unoriented. That is,the process is a strong draw process for the first material and a weakdraw process for the second material. As an example of strong and weakflows, let us consider PEN of approximately 0.48 IV, an initial drawrate of about 15% per second, and a uniaxial draw profile that increasesthe draw ratio in a linear manner to a final draw ratio of 6.0. At a webtemperature of about 155° C., PEN experiences weak flow that leaves itin a state of low birefringence. At 135° C., PEN experiences a strongflow that makes it highly birefringent. The degree of orientation andcrystallization increases in this strong flow regime as the temperaturedrops further. These values are for illustration only and should not betaken as the limiting values of these regimes.

[0127] More general ranges for material selection can be understood byconsidering the more general case of polyesters. For PET, approximatevalues for the WLF parameters can be taken as c₁=11.5, c₂=55.2 andT₀=T_(g)+4° C.=80° C. These values are for purposes of illustrationonly, it being understood that empirical determination of theseconstants may give somewhat varying results. For example, alternatevalues using the “universal” values of c₁=17.7 and c₂=51.6, and usingT₀=85° C., have been proposed. At a temperature 20° C. above the glasstransition, the effect of a 5° C. increase/decrease in temperature is todecrease/increase the characteristic time and Ws by a factor of four. At10° C. above the glass transition, the effect is much stronger, about afactor of ten. For PEN, T₀ is estimated as approximately 127° C. ForDMI-based polyester (e.g. PEI), T₀ is estimated as about 64° C. Theglass transition of polyester with some higher alkane glycol such ashexane diol might be expected, based on these example WLF values, tohave a 1° C. decrease in glass transition for every 1% replacement ofethylene glycol. For coPEN, the glass transition can be estimated usingthe so-called Fox equation. The reciprocal of the coPEN glass transitiontemperature (in absolute degrees) is equal to the linear,compositionally weighted average of its component reciprocal glasstransition temperatures (in absolute degrees). Therefore, a coPEN of 70%naphthalene dicarboxylate (NDC) and 30% dimethylterephthalate (DMT)would have an estimated glass transition of about 107° C., assumingglass transitions for PEN and PET of 123° C. and 76° C., respectively.Likewise, a coPEN of 70% NDC and 30% DMI would have a glass transitionof about 102° C. Roughly, the latter coPEN would be expected toexperience a weak flow at a temperature 20° C. lower than that requiredfor weak flow for PEN, under the same conditions. Thus, at webtemperatures of 135° C., coPEN is weakly oriented and PEN is stronglyoriented under the process conditions cited. This particular choice ofresins has been previously cited as one example of a preferredembodiment for multilayer reflective polarizers in WO 95/17303.

[0128] The temperature effects the strength of the flow secondarily byaltering the rate of nucleation and crystal growth. In the undrawnstate, there is a temperature of maximum crystallization rate. Rates areslowed below this temperature due to much slower molecular motions ascharacterized by the relaxation times. Above this temperature, the ratesare slowed by the decrease in the degree of supercooling (the meltingtemperature minus the process temperature), which is related to thethermodynamic driving force for crystallization. If the draw is fast andthe temperature is near T_(g), the onset of strain inducedcrystallization may be enhanced (making the draw still stronger) byraising the temperature, because little additional relaxation occurs atthe higher temperature but nucleation and growth can be accelerated. Ifthe temperature of draw is near the melting point, raising the drawtemperature and thus decreasing the degree of supercooling may decreasethe rate of strain-induced crystallization, delaying the onset of suchcrystallization and thereby making the flow effectively weaker. Amaterial can be deliberately designed to have a low melting point andthus little or no supercooling. Copolymers are known to have a reducedmelting point due to the impurity effect of the additional monomer. Thiscan be used effectively to maintain the second polymer in a state of loworientation.

[0129] The aforementioned effect of melting point can also be used toaccomplish the third method for obtaining draw compatibility in the caseof a second material with desired isotropy. Alternatively, this may beused after a drawing step during a multiple drawing process to achieveisotropy in one or more of the materials. Drawing processes that arestrong for both the first and second material may be used as long as theeffects of that draw can be eliminated in the second polymer in asubsequent step. For example, a heat setting step can be used toaccomplish relaxation of an oriented, but still amorphous, secondpolymer. Likewise, a heat setting step can be used to melt an orientedand crystallized second polymer, as long as it is adequately quenched.

[0130] Heat setting can also be useful in improving other properties,such as dimensional stability (with regard to both temperature andhumidity) and interlayer adhesion. Finally, tension conditions atquenching, prior to winding, can also affect physical properties, suchas shrinkage. Reduced winding tension and reduced cross web tension viaa toe in (reduction in transverse draw ratio) can reduce shrinkage in avariety of multilayer optical films. Post-winding heat treatment of filmrolls can also be used to improve dimensional stability and reduceshrinkage.

[0131] In general, the birefringence of a polymer experiencing a strongflow deformation tends to increase with the draw ratio. Because ofstrain-induced crystallization, for a given draw process there may be acritical draw ratio at which this birefringence begins to increase moredramatically. After onset of crystallization, the slope may again change(e.g. drop) due to changes in the relative amount of continuednucleation and growth with further drawing. For the inventive multilayeroptical films, the increase in the birefringence of at least one of thepolymers leads to an increase in the reflection of light of wavelengthsappropriate to the layer thicknesses of the multilayer stack. Thisreflective power also tends to increase in relative measure to theorientation.

[0132] On the other hand, adhesion between layers in the multilayerstack is often adversely affected by drawing, with stretched filmsfrequently being much more prone to exfoliation of layers than the castwebs from which they were made. Surprisingly, this decrease ininterlayer adhesion, as discovered by the present inventors, may alsoexperience a critical point under some process/material combinations sothat the majority of the decrease happens relatively abruptly as aspecific draw ratio is exceeded. This critical change need not correlatewith changes in the birefringence. In other cases, the behavior can benon-linear but not necessarily abrupt. The existence and value of thiscritical draw ratio is likely a complex function of the polymersinvolved and a host of other process conditions, and needs to bedetermined on a case-by-case basis. The compromise between high opticalextinction and high interlayer adhesion with respect to draw ratio willbe dominated by the existence and location of an abrupt transition orother functional form, e.g., with the optimal draw ratio for a givenfilm likely to be selected from the maximum possible draw ratio and thedraw ratio just below the abrupt interlayer adhesion transition.

[0133] There are other process compromises that may be apparent forparticular resin system choices. For instance, in certain systems,higher draw ratio may also result in higher off-angle color. Increasedoff-angle color can result from an increase in the z-index (theout-of-plane index) interlayer mismatch due to the lowering of thez-index of refraction of the first material (such as PEN), while thesecond material z-index remains nearly constant. The drop in z-indicesin aromatic polyesters may be related to the planarization of thecrystals within the film, which causes the planes of the aromatic ringsto tend to lie in the plane of the film. Such compromises may sometimesbe avoided by altering the selection of resin pairs. For example,reducing the level of crystallinity while maintaining a given level oforientation may improve both interlayer adhesion and off-angle colorwithout reducing extinction power, as long as the difference between therefractive index of the in-plane draw direction and the in-planenon-drawn direction remains about the same. This latter condition can bemet by using high NDC content coPENs as the first polymer. The lowermelting points of these polymers suggest that lower levels ofcrystallinity would be obtained at the same level of orientation,allowing extinction to be maintained while decreasing off-angle colorand possibly increasing interlayer adhesion. It will be appreciated thatsimilar process considerations would pertain to additional materials,such as those to be used in the skin and/or PBLs. If these materials areto be isotropic, thus avoiding polarization retardation from thickbirefringent layers, they should be chosen in accord with therequirements of a second polymer with desired isotropy.

[0134] Finally, the need for careful control and uniformity of processconditions should be appreciated to form high quality optical films inaccordance with the present invention. Draw uniformity is stronglyinfluenced by temperature, and thus uniform temperature is typicallydesired for a uniform film. Likewise, caliper (thickness) andcompositional uniformity is also desirable. One preferred method toobtain uniformity is to cast a flat uniform film, which is thenuniformly drawn to make a uniform final film. Often, final filmproperties are more uniform (in off-angle color, for example) and better(e.g. interlayer adhesion) under such processes. Under certaincircumstances, cast thickness profiling can be used to compensate foruneven drawing to produce a final film of uniform caliper. In addition,infrared edge heating, discussed above, can be used in conjunction withcast thickness profiling.

[0135] Film Uniformity

[0136] The high quality multilayer optical films and other opticaldevices made in accordance with the present invention can be made so asto exhibit a degree of physical and optical uniformity over a large areathat far exceeds that accessible with prior art films. In accordancewith the method of the invention, the distortions of layer thickness andoptical caliper encountered in prior art cast (not drawn) films isavoided by biaxially stretching the cast web by a factor of betweenabout 2×2 and about 6×6, and preferably about 4×4. These ranges tend tomake the lateral layer thickness variations, and therefore the colorvariations, much less abrupt. Furthermore, because the film is made bystretching a cast web (as opposed to casting a finished film directlywithout stretching), the narrower the cast web width, the fewer thedistortions in layer thickness distribution in the extrusion die becauseof significantly less layer spreading occurring in the narrower die.

[0137] Many other process considerations discussed in the sections aboveand intended to improve layer thickness uniformity also improve thecolor uniformity, as color depends directly on layer thickness. Theseinclude, but are not limited to, multilayer resin system rheologicalmatching, filtration, feedblock design, multiplier design, die design,PBL and skin layer selection, temperature control, electrostatic pinningparameters, use of web thickness variation scanning devices, use of acasting nip roll, vibration control, and web edge heating in the tenter.

[0138] Errors in extrusion equipment design and machining, and in theextrusion control, will lead to both systematic and random thicknesserrors. For uniform color films in general, the random errors can leadto both down web and cross web variations in color, and the systematicerrors, although not changing, will affect both the overall color of thefilm and the crossweb color variation.

[0139] Both random and systematic errors can occur for the overall filmcaliper as well as for individual layers. Overall film caliper errorsare most easily detected and monitored via the optical transmission orreflectance spectra. Thus, an on-line spectrophotometer can be set up tomeasure the spectral transmission of the film as it comes off the line,thereby providing the necessary information to measure color uniformityand provide feedback for process controls. Individual layer errors mayor may not affect the perceived color, depending mostly on where theyare in the optical stack and on the magnitude of the errors.

[0140] Systematic errors are repeatable deviations from the designthickness for any or all layers in the stack. They can occur because ofdesign approximations inherent in the polymer flow model used to designthe multipliers and feedblock, or because of machining errors in thefeedblock and die. These errors can be eliminated by redesign andre-machining until the errors are reduced to design criteria. Theseerrors can also be reduced by machining a feedblock that will producethe required number of layers in the optical film without resort to amultiplier.

[0141] Random errors can be caused by: (1) fluctuations in feedblock anddie zone temperatures, (2) resin non-homogeneity, (3) improper controlof melt temperatures through the melt train, which selectively degradeparts of the melt stream, (4) contamination of the feedblock or die dueto degraded resin, (5) process control errors such as melt pressure,temperature and pumping rate variations, and (6) hydrodynamic flowinstabilities. The flow modeling should provide input to the feedblockand die designs in order to avoid conditions that could cause such flowinstabilities.

[0142] Overall thickness uniformity is affected by die design, castingwheel speed fluctuations, system vibrations, die gap control,electrostatic pinning, and film stretching conditions. These variationscan be either random or systematic. Systematic errors do not necessarilygive a constant (e.g., unchanging) color. For example, vibrations of thedie or casting wheel can cause a repeating spatial color variation witha periodicity on the order of 0.5 to 50 cm. In certain applications suchas decorative film, where a periodic spatial color variation may bedesirable in the finished film, controlled periodic vibrations may beintentionally imparted to the casting wheel. However, where coloruniformity is desired and good thickness control is essential, thecasting wheel is fitted with a direct drive motor (e.g., no gearreduction). One example of such a motor is a D.C. brush servo motor,such as part number TT-10051A, available commercially from Kollmorgan.Higher speed motors with gear reduction can be used, but a high qualitysystem with proper electrical tuning and a smooth gearbox is essential.System vibrations, particularly of the die relative to the castingwheel, can be minimized by placing the casting station on concrete padson the ground floor of the casting installation. Other means ofdampening or isolation will be apparent to one skilled in the mechanicalarts.

[0143] The sources of vibrations can be identified with the help of aweb thickness variation scanning device discussed earlier. If the periodof an oscillation can be identified from the output of such a device, asearch may be made for process elements, or even extraneous sources,which exhibit oscillatory behavior of identical period. These units canthen be made more rigid, vibration-damped, or vibration-isolated fromthe die and casting wheel by methods known in the art, or may simply beturned off or relocated if not essential to the process. Hence, avibration identified by periodicity as being due to the rotation of theextruder screw could be isolated, for example, by the use of a dampingmaterial between the extruder gate and the neck tube, while a vibrationidentified by periodicity as being due to a room fan could be removed byturning off or relocating the fan. In addition, a vibration of the dieor casting station which cannot be totally eliminated can be preventedfrom resulting in vibratory relative motion between the die and castingstation by mechanically linking the die to the casting station via someform of rigid superstructure. Many designs for such avibration-communicating mechanical linkage will be apparent.Furthermore, when strain hardening materials are employed in the film,stretching should be performed at sufficiently low temperatures toproduce a uniform stretch across the web, and the pinning wire should berigidly mounted.

[0144] Additional control over layer thickness and optical caliper isachieved through the use of a precision casting wheel drive mechanismhaving a constant rotation speed. The casting wheel is designed andoperated such that it is free of vibrations that would otherwise causeweb thickness “chatter” and subsequent layer thickness variations in thedown-web direction. Applicants have found that those vibrations whichproduce a relative motion between the die and casting wheel result ineffective speed variations in the casting wheel as it draws out theextrudate coming from the die. These speed variations cause modulationsin film caliper and optical layer thickness that are particularlypronounced in the strain-hardening materials advantageously employed inmaking the optical films of the present invention, resulting in colorvariations across the surface of the film. Accordingly, absent thesecontrols at the casting wheel, the normal vibrations encountered in theextrusion process are sufficient to noticeably diminish color uniformityin the optical films of the present invention. The methods of thepresent invention have allowed the production, for the first time, ofcolor shifting films made from polymeric materials that have a highdegree of color uniformity at any particular viewing angle. Thus, filmsmay be made in accordance with the method of the present invention inwhich the desired bandwidth of light transmitted or reflected at aparticular angle of incidence varies by less than about 1 or 2 nm overan area of at least 10 cm², and more preferably, at least 100 cm², andin which the absolute bandedges of the spectral reflectance peaks varyin wavelength by less than about +/−4 nm.

[0145] While thickness and/or color uniformity is important in manyapplications of the films of the present invention, in otherapplications, such as decorative films, color uniformity may be eitherunimportant or undesirable. In applications where color variations aredesirable, they may be intentionally imparted to the inventive opticalfilms by inducing thickness variations of a desired spatial frequencyacross or along a portion of the web at any point prior to quenching ofthe web in such a manner as to result in modulations in the thickness ofthe optical stack. While there are numerous ways of accomplishing thiseffect (e.g., by inducing vibrations in the casting wheel), suchmodulations may be conveniently imparted by inducing vibrations of adesired frequency (or frequencies) in the pinning wire. For example, byinducing a vibration on the pinning wire, the color of a polarizer filmwas periodically varied, in straight lines across the film, from aneutral gray transmission color to a red color. The red stripes were 6.0mm apart in the downweb direction. Calculated frequency of the pinningwire vibration was 21 Hz.

[0146] Local random color variations can also be achieved by extrudingfilms of the present invention with small internal bubbles to produceattractive decorative effects. Bubbles can be created by several methodsincluding not drying the resin as sufficiently as one would normally do,or by slightly overheating a thermally sensitive resin such as PMMA tocreate a similar effect. The small bubbles formed locally distort themicrolayers and cause a local color change that can give the appearanceof depth in some instances.

[0147] Although the methods described above for inducing colorvariations appear to teach a nonuniform film, the starting base filmhaving uniform color with high stop band reflectivity and high colorsaturation, although locally disrupted by a given method, may bedesirable in controlling the average hue, color saturation, andbrightness of such a decorative film. The local color variations taughthere are more noticeable when applied to a uniform color shifting filmhaving reflection bands with inherently high reflectivity and bandedgeswith high slopes.

[0148] As noted above, vibrations in the casting wheel cause the speedof the casting wheel to fluctuate, resulting in variations of layerthicknesses in the film. The frequency (or frequencies) of thevibrations can be modulated to impart repeating sequences or patterns ofcolors to the resulting film.. Furthermore, these color variations canbe accomplished without destroying the color shifting characteristicstypical of the films of the present invention, thereby allowing theproduction of colorful films (often spanning the entire visiblespectrum) in which the colors appear to shimmer or move as the angle ofincidence is varied

[0149] Periodic color variations may also be imparted to the film byembossing it with a pattern. Due in part to the fact that the embossedportion is no longer coplanar with the rest of the film, it will exhibita different color or colors than the rest of the film. Thus, strikingeffects have been produced by embossing the color shifting films of thepresent invention with, for example, a fishnet pattern (e.g., gold on ared background) or an emblem.

[0150] In certain instances, similar principles may be used to remove ortune out periodic color variations in the film, thereby improving thecolor uniformity of the film. Thus, where a source is found to impartvibrations of a given frequency or a given periodic frequency to theweb, vibrations of equal amplitude (but opposite phase) can be impartedto the web (e.g., through the casting wheel), resulting in destructiveinterference and effective removal of the source from the process.

[0151] Additional Layers and Coatings

[0152] As further steps in the process of making the high qualitycoextruded polymeric multilayer optical films of the present invention,various layers or coatings may be applied to at least a portion of oneor both sides of the multilayer optical stack to modify or enhance thephysical, chemical, or optical characteristics of the film. These layersor coatings may be integrated at the time of film formation, either bycoextrusion or in a separate coating or extrusion step, or they may beapplied to the finished optical film at a later time. Examples ofadditional layers or coatings are described in U.S. patent applicationSer. No. 08/910,660 filed Aug. 13, 1997 entitled “Multilayer PolymerFilm with Additional Coatings or Layers” which is incorporated herein byreference. A non-limiting listing of coatings or layers that may becombined with the coextruded multilayer film is described in more detailin the following examples.

[0153] A non-optical layer of material may be coextensively disposed onone or both major surfaces of the film, i.e., the extruded opticalstack. The composition of the layer, also called a skin layer, may bechosen, for example, to protect the integrity of the optical layers, toadd mechanical or physical properties to the final film or to addoptical functionality to the final film. Suitable materials of choicemay include the material of one or more of the optical layers. Othermaterials with a melt viscosity similar to the extruded optical layersmay also be useful. It should also be noted that many of the mechanicaladvantages derived from skin layers can also be derived from ananalogous internal thick non-optical layer, e.g. a PBL.

[0154] A skin layer or layers may reduce the wide range of shearintensities the extruded multilayer stack might experience within theextrusion process, particularly at the die. A high shear environment maycause undesirable deformations in the optical layers. A skin layer orlayers may also add physical strength to the resulting composite orreduce problems during processing, such as, for example, reducing thetendency for the film to split during the orientation process. Skinlayer materials that remain amorphous can result in films having ahigher toughness, while skin layer materials that are semicrystallinecan result in films having a higher tensile modulus. Other functionalcomponents such as antistatic additives, UV absorbers, dyes,antioxidants, and pigments, may be added to the skin layer, providedthey do not substantially interfere with the desired optical propertiesof the resulting product. Skin layers or coating may also be used to aidin post-extrusion processing; for example, by preventing sticking of thefilm to hot rollers or tenter clips.

[0155] Skin layers or coatings may also be added to impart desiredbarrier properties to the resulting film or device. Thus, for example,barrier films or coatings may be added as skin layers, or as a componentin skin layers, to alter the transmissive properties of the film ordevice towards liquids, such as water or organic solvents, or gases,such as oxygen or carbon dioxide.

[0156] Skin layers or coatings may also be added to impart or improveabrasion resistance in the resulting article. Thus, for example, a skinlayer comprising particles of silica embedded in a polymer matrix may beadded to an optical film produced in accordance with the invention toimpart abrasion resistance to the film. Such a skin layer, however,should not unduly compromise the optical properties required for theapplication to which the film is directed.

[0157] Skin layers or coatings may also be added to impart or improvepuncture and/or tear resistance in the resulting article. Thus, forexample, in embodiments in which the outer layer of the optical filmcontains coPEN, a skin layer of monolithic coPEN may be coextruded withthe optical layers to impart good tear resistance to the resulting film.Factors to be considered in selecting a material for a tear resistantlayer include percent elongation to break, Young's modulus, tearstrength, adhesion to interior layers, percent transmittance andabsorbance in an electromagnetic bandwidth of interest, optical clarityor haze, refractive indices as a function of frequency, texture androughness, melt thermal stability, molecular weight distribution, meltrheology and coextrudability, miscibility and rate of inter-diffusionbetween materials in the skin and optical layers, viscoelastic response,relaxation and crystallization behavior under draw conditions, thermalstability at use temperatures, weatherability, ability to adhere tocoatings and permeability to various gases and solvents. Puncture ortear resistant skin layers may be applied during the manufacturingprocess or later coated onto or laminated to the optical film. Adheringthese layers to the optical film during the manufacturing process, suchas by a coextrusion process, provides the advantage that the opticalfilm is protected during the manufacturing process. In some embodiments,one or more puncture or tear resistant layers may be provided within theoptical film, either alone or in combination with a puncture or tearresistant skin layer.

[0158] The skin layers may be applied to one or two sides of theextruded optical stack at some point during the extrusion process, i.e.,before the extruded and skin layer(s) exit the extrusion die. This maybe accomplished using conventional coextrusion technology, which mayinclude using a three-layer coextrusion die. Lamination of skin layer(s)to a previously formed multilayer film is also possible. Total skinlayer thicknesses may range from about 2% to about 50% of the totaloptical stack/skin layer thickness.

[0159] In some applications, additional layers may be coextruded oradhered on the outside of the skin layers during manufacture of theoptical films. Such additional layers may also be extruded or coatedonto the optical film in a separate coating operation, or may belaminated to the optical film as a separate film, foil, or rigid orsemi-rigid substrate such as polyester (PET), acrylic (PMMA),polycarbonate, metal, or glass.

[0160] Many polymers are suitable for skin layers. Of the predominantlyamorphous polymers, suitable examples include copolyesters based on oneor more of terephthalic acid, 2,6-naphthalene dicarboxylic acid,isophthalic acid phthalic acid, or their alkyl ester counterparts, andalkylene diols, such as ethylene glycol. Examples of semicrystallinepolymers suitable for use in skin layers include 2,6-polyethylenenaphthalate, polyethylene terephthalate, and nylon materials. Skinlayers that may be used to increase the toughness of the optical filminclude high elongation polyesters such as ECDEL™ and PCTG 5445(available commercially from Eastman Chemical Co., Rochester, N.Y.) andpolycarbonates. Polyolefins, such as polypropylene and polyethylene, mayalso be used for this purpose, especially if they are made to adhere tothe optical film with a compatibilizer.

[0161] Various functional layers or coatings may be added to the opticalfilms and devices of the present invention to alter or improve theirphysical or chemical properties, particularly along the surface of thefilm or device. Such layers or coatings may include, for example, slipagents, low adhesion backside materials, conductive layers, antistaticcoatings or films, barrier layers, flame retardants, UV stabilizers,abrasion resistant materials, optical coatings, or substrates designedto improve the mechanical integrity or strength of the film or device.

[0162] The optical films of the present invention may comprise a slipagent that is incorporated into the film or added as a separate coatingin order to improve roll formation and convertibility of the film. Inmost applications, slip agents will be added to only one side of thefilm, ideally the side facing the rigid substrate in order to minimizehaze. The films and optical devices of the present invention may begiven good slip properties by treating them with low friction coatingsor slip agents, such as polymer beads coated onto the surface.Alternately, the morphology of the surfaces of these materials may bemodified, as through manipulation of extrusion conditions, to impart aslippery surface to the film; methods by which surface morphology may beso modified are described in U.S. Ser. No. 08/612,710.

[0163] The films and other optical devices made in accordance with theinvention may also be provided with one or more adhesives to laminatethe optical films and devices of the present invention to another film,surface, or substrate. Such adhesives include both optically clear anddiffuse adhesives, as well as pressure sensitive and non-pressuresensitive adhesives. Pressure sensitive adhesives are normally tacky atroom temperature and can be adhered to a surface by application of, atmost, light finger pressure, while non-pressure sensitive adhesivesinclude solvent, heat, or radiation activated adhesive systems. Examplesof adhesives useful in the present invention include those based ongeneral compositions of polyacrylate; polyvinyl ether; diene-containingrubbers such as natural rubber, polyisoprene, and polyisobutylene;polychloroprene; butyl rubber; butadiene-acrylonitrile polymers;thermoplastic elastomers; block copolymers such as styrene-isoprene andstyrene-isoprene-styrene block copolymers, ethylene-propylene-dienepolymers, and styrene-butadiene polymers; polyalphaolefins; amorphouspolyolefins; silicone; ethylene-containing copolymers such as ethylenevinyl acetate, ethylacrylate, and ethylmethacrylate; polyurethanes;polyamides; polyesters; epoxies; polyvinylpyrrolidone andvinylpyrrolidone copolymers; and mixtures of the above. Additionally,the adhesives can contain additives such as tackifiers, plasticizers,fillers, antioxidants, stabilizers, pigments, diffusing particles,curatives, and solvents. In some applications, as where the opticalfilms of the present invention are to be used as a component in adhesivetapes, it may be desirable to treat the films with low adhesion backsize(LAB) coatings or films such as those based on urethane, silicone orfluorocarbon chemistry. Films treated in this manner will exhibit properrelease properties towards pressure sensitive adhesives (PSAs), therebyenabling them to be treated with adhesive and wound into rolls Adhesivetapes, sheets, or die-cuts made in this manner can be used fordecorative purposes or in any application where a diffusely reflectiveor transmissive surface on the tape is desirable. When a laminatingadhesive is used to adhere an optical film of the present invention toanother surface, the adhesive composition and thickness are preferablyselected so as not to interfere with the optical properties of theoptical film. For example, when laminating additional layers to anoptical polarizer or mirror wherein a high degree of transmission isdesired, the laminating adhesive should be optically clear in thewavelength region that the polarizer or mirror is designed to betransparent in.

[0164] The films and optical devices of the present invention may alsobe provided with one or more conductive layers. Such conductive layersmay comprise metals such as silver, gold, copper, aluminum, chromium,nickel, tin, and titanium, metal alloys such as silver alloys, stainlesssteel, and inconel, and semiconductor metal oxides such as doped andundoped tin oxides, zinc oxide, and indium tin oxide (ITO).

[0165] The films and optical devices of the present invention may alsobe provided with antistatic coatings or films. Such coatings or filmsinclude, for example, V₂O₅ and salts of sulfonic acid polymers, carbonor other conductive metal layers.

[0166] The optical films and devices of the present invention may alsobe provided with one or more barrier films or coatings that alter thetransmissive properties of the optical film towards certain liquids orgases. Thus, for example, the devices and films of the present inventionmay be provided with films or coatings that inhibit the transmission ofwater vapor, organic solvents, O₂, or CO₂ through the film. Barriercoatings will be particularly desirable in high humidity environments,where components of the film or device would be subject to distortiondue to moisture permeation.

[0167] The optical films and devices of the present invention may alsobe treated with flame retardants, particularly when used inenvironments, such as on airplanes, that are subject to strict firecodes. Suitable flame retardants include aluminum trihydrate, antimonytrioxide, antimony pentoxide, and flame retarding organophosphatecompounds.

[0168] The optical films and devices of the present invention may alsobe provided with abrasion-resistant or hard coatings, which willfrequently be applied as a skin layer. These include acrylic hardcoatssuch as Acryloid A-11 and Paraloid K-120N, available from Rohm & Haas,Philadelphia, Pa.; urethane acrylates, such as those described in U.S.Pat. No. 4,249,011 and those available from Sartomer Corp., Westchester,Pa.; and urethane hardcoats obtained from the reaction of an aliphaticpolyisocyanate (e.g., Desmodur N-3300, available from Miles, Inc.,Pittsburgh, Pa.) with a polyester (e.g., Tone Polyol 0305, availablefrom Union Carbide, Houston, Tex.).

[0169] The optical films and devices of the present invention mayfurther be laminated to rigid or semi-rigid substrates, such as, forexample, glass, metal, acrylic, polyester, and other polymer backings toprovide structural rigidity, weatherability, or easier handling. Forexample, the optical films of the present invention may be laminated toa thin acrylic or metal backing so that it can be stamped or otherwiseformed and maintained in a desired shape. For some applications, such aswhen the optical film is applied to other breakable backings, anadditional layer comprising PET film or puncture-tear resistant film maybe used. Additionally, for some applications such as in liquid crystaldisplays, the multilayer optical film may be combined with a lightredirecting structure as described in U.S. patent application Ser. No.08/402,349 filed Mar. 10, 1995. Such a light redirecting structurecoated onto the multilayer optical film, laminated as a separated film,cast and cured on a multilayer optical film substrate, or embosseddirectly onto the surface of the multilayer optical film.

[0170] The optical films and devices of the present invention may alsobe provided with shatter resistant films and coatings. Films andcoatings suitable for this purpose are described, for example, inpublications EP 592284 and EP 591055, and are available commerciallyfrom 3M Company, St. Paul, Minn.

[0171] Various optical layers, materials, and devices may also beapplied to, or used in conjunction with, the films and other opticaldevices of the present invention for specific applications. Theseinclude, but are not limited to, magnetic or magneto-optic coatings orfilms; liquid crystal panels, such as those used in display panels andprivacy windows; photographic emulsions; fabrics; prismatic films, suchas linear Fresnel lenses; brightness enhancement films; holographicfilms or images; embossable films; anti-tamper films or coatings; IRtransparent film for low emissivity applications; release films orrelease coated paper; and polarizers or mirrors. Multiple additionallayers on one or both major surfaces of the optical film arecontemplated, and can be any combination of aforementioned coatings orfilms. For example, when an adhesive is applied to the optical film, theadhesive may contain a white pigment such as titanium dioxide toincrease the overall reflectivity, or it may be optically transparent toallow the reflectivity of the substrate to add to the reflectivity ofthe optical film.

[0172] The films and other optical devices made in accordance with theinvention may include one or more anti-reflective layers or coatings,such as, for example, conventional vacuum coated dielectric metal oxideor metal/metal oxide optical films, silica sol gel coatings, and coatedor coextruded antireflective layers such as those derived from low indexfluoropolymers such as THV™, an extrudable fluoropolymer available from3M Company (St. Paul, Minn.). Such layers or coatings, which may or maynot be polarization sensitive, serve to increase transmission and toreduce reflective glare, and may be imparted to the films and opticaldevices of the present invention through appropriate surface treatment,such as coating or sputter etching. In some embodiments of the presentinvention, it is desired to maximize the transmission and/or minimizethe specular reflection for certain polarizations of light. In theseembodiments, the optical body may comprise two or more layers in whichat least one layer comprises an anti-reflection system in close contactwith a layer providing the continuous and disperse phases. Such ananti-reflection system acts to reduce the specular reflection of theincident light and to increase the amount of incident light that entersthe portion of the body comprising the continuous and disperse layers.Such a function can be accomplished by a variety of means well known inthe art. Examples are quarter wave anti-reflection layers, two or morelayer anti-reflective stack, graded index layers, and graded densitylayers. Such anti-reflection functions can also be used on thetransmitted light side of the body to increase transmitted light ifdesired.

[0173] The films and other optical devices made in accordance with theinvention may also be provided with a film or coating which impartsanti-fogging properties. In some cases, an anti-reflection layer asdescribed above will serve the dual purpose of imparting bothanti-reflection and anti-fogging properties to the film or device.Various anti-fogging agents are known to the art which are suitable foruse with the present invention. Typically, however, these materials willsubstances, such as fatty acid esters, which impart hydrophobicproperties to the film surface and which promote the formation of acontinuous, less opaque film of water. Several inventors have reportedcoatings that reduce the tendency for surfaces to “fog”. For example,U.S. Pat. No. 3,212,909 to Leigh discloses the use of ammonium soap,such as alkyl ammonium carboxylates in admixture with a surface activeagent which is a sulfated or sulfonated fatty material, to produce aanti-fogging composition. U.S. Pat. No. 3,075,228 to Elias discloses theuse of salts of sulfated alkyl aryloxypolyalkoxy alcohol, as well asalkylbenzene sulfonates, to produce an anti-fogging article useful incleaning and imparting anti-fogging properties to various surfaces. U.S.Pat. No. 3,819,522 to Zmoda, discloses the use of surfactantcombinations comprising derivatives of decyne diol as well as surfactantmixtures which include ethoxylated alkyl sulfates in an anti-foggingwindow cleaner surfactant mixture. Japanese Patent Kokai No. Hei6[1994]41,335 discloses a clouding and drip preventive compositioncomprising colloidal alumina, colloidal silica and an anionicsurfactant. U.S. Pat. No. 4,478,909 (Taniguchi et al) discloses a curedanti-fogging coating film which comprises polyvinyl alcohol, a finelydivided silica, and an organic silicon compound, the carbon/siliconweight ratio apparently being important to the film's reportedanti-fogging properties. Various surfactants, includefluorine-containing surfactants, may be used to improve the surfacesmoothness of the coating. Other anti-fog coatings incorporatingsurfactants are described in U.S. Pat. Nos. 2,803,552; 3,022,178; and3,897,356. World Patent No. PCT 96/18,691 (Scholtz et al) disclosesmeans by which coatings may impart both anti-fog and anti-reflectiveproperties.

[0174] The films and optical devices of the present invention may alsobe protected from UV radiation through the use of UV stabilized films orcoatings. Suitable UV stabilized films and coatings include those whichincorporate benzotriazoles or hindered amine light stabilizers (HALS)such as Tinuvin™ 292, both of which are available commercially from CibaGeigy Corp., Hawthorne, N.Y. Other suitable UV stabilized films andcoatings include those which contain benzophenones or diphenylacrylates, available commercially from BASF Corp., Parsippany, N.J. Suchfilms or coatings will be particularly important when the optical filmsand devices of the present invention are used in outdoor applications orin luminaires where the source emits significant amount of light in theUV region of the spectrum.

[0175] The films and optical devices of the present invention may alsoinclude antioxidants such as, for example,4,4′-thiobis-(6-t-butyl-m-cresol),2,2′-methylenebis-(4-methyl-6-t-butyl-butylphenol),octadecyl-3,5-di-t-butyl-4-hydroxyhydrocinnamate,bis-(2,4-di-t-butylphenyl) pentaerythritol diphosphite, Irganox™ 1093(1979)(((3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl)methyl)-dioctadecylester phosphonic acid), Irganox™ 1098(N,N′-1,6-hexanediylbis(3,5-bis(1,1-dimethyl)-4-hydroxy-benzenepropanamide),Naugaard™ 445 (aryl amine), Irganox™ L 57 (alkylated diphenylamine),Irganox™ L 115 (sulfur containing bisphenol), Irganox™ LO 6 (alkylatedphenyl-delta-napthylamine), Ethanox 398 (flourophosphonite), and2,2′-ethylidenebis(4,6-di-t-butylphenyl)fluorophosnite. A group ofantioxidants that are especially preferred are sterically hinderedphenols, including butylated hydroxytoluene (BHT), Vitamin E(di-alpha-tocopherol), Irganox™ 1425WL(calciumbis-(O-ethyl(3,5-di-t-butyl-4-hydroxybenzyl))phosphonate), Irganox™ 1010(tetrakis(methylene(3,5,di-t-butyl-4-hydroxyhydrocinnamate))methane),Irganox™ 1076 (octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate),Ethanox™ 702 (hindered bis phenolic), Etanox 330 (high molecular weighthindered phenolic), and Ethanox™ 703 (hindered phenolic amine).

[0176] The films and optical devices of the present invention may alsobe treated with inks, dyes, or pigments to alter their appearance or tocustomize them for specific applications. Thus, for example, the filmsmay be treated with inks or other printed indicia such as those used todisplay product identification, advertisements, warnings, decoration, orother information. Various techniques can be used to print on the film,such as screen printing, letterpress, offset, flexographic printing,stipple printing, laser printing, and so forth, and various types of inkcan be used, including one and two component inks, oxidatively dryingand UV-drying inks, dissolved inks, dispersed inks, and 100% inksystems. The appearance of the optical film or other optical device mayalso be altered by coloring the device such as by laminating a dyed filmto the optical device, applying a pigmented coating to the surface ofthe optical device, or including a pigment in one or more of thematerials used to make the optical device. Both visible and near IR dyesand pigments are contemplated in the present invention, and include, forexample, optical brighteners such as dyes that absorb in the UV andfluoresce in the visible region of the color spectrum. Other additionallayers that may be added to alter the appearance of the optical filminclude, for example, opacifying (black) layers, diffusing layers,holographic images or holographic diffusers, and metal layers. Each ofthese may be applied directly to one or both surfaces of the opticalfilm, or may be a component of a second film or foil construction thatis laminated to the optical film. Alternately, some components such asopacifying or diffusing agents, or colored pigments, may be included inan adhesive layer which is used to laminate the optical film to anothersurface.

[0177] The films and devices of the present invention may also beprovided with metal coatings. Thus, for example, a metallic layer may beapplied directly to the optical film by pyrolysis, powder coating, vapordeposition, cathode sputtering, ion plating, and the like. Metal foilsor rigid metal plates may also be laminated to the optical film, orseparate polymeric films or glass or plastic sheets may be firstmetallized using the aforementioned techniques and then laminated to theoptical films and devices of the present invention.

[0178] Dichroic dyes are a particularly useful additive for many of theapplications to which the films and optical devices of the presentinvention are directed, due to their ability to absorb light of aparticular polarization when they are molecularly aligned within thematerial. When used in a film or other optical body, the dichroic dyecauses the material to absorb one polarization of light more thananother. Suitable dichroic dyes for use in the present invention includeCongo Red (sodium diphenyl-bis-α-naphthylamine sulfonate), methyleneblue, stilbene dye (Color Index (CI)=620), and 1,1′-diethyl-2,2′-cyaninechloride (CI=374 (orange) or CI=518 (blue)). The properties of thesedyes, and methods of making them, are described in E. H. Land, ColloidChemistry (1946). These dyes have noticeable dichroism in polyvinylalcohol and a lesser dichroism in cellulose. A slight dichroism isobserved with Congo Red in PEN. Still other dichroic dyes, and methodsof making them, are discussed in the Kirk Othmer Encyclopedia ofChemical Technology, Vol. 8, pp. 652-661 (4th Ed. 1993), and in thereferences cited therein. Dychroic dyes in combination with certainpolymer systems exhibit the ability to polarize light to varyingdegrees. Polyvinyl alcohol and certain dichroic dyes may be used to makefilms with the ability to polarize light. Other polymers, such aspolyethylene terephthalate or polyamides, such as nylon-6, do notexhibit as strong an ability to polarize light when combined with adichroic dye. The polyvinyl alcohol and dichroic dye combination is saidto have a higher dichroism ratio than, for example, the same dye inother film forming polymer systems. A higher dichroism ratio indicates ahigher ability to polarize light. Combinations of a dichroic dye with amultilayer optical polarizer are described in U.S. patent applicationsSer. No. 08/402,042 entitled “Optical Polarizer” filed Mar. 10, 1995; inSer. No. 09/006,458 entitled “Dichroic Polarizing Film and OpticalPolarizers Containing the Film” filed Jan. 13, 1998; and in Ser. No.09/006,468 entitled “Optical Device with a Dichroic Polarizer and aMultilayer Optical Film” filed Jan. 13, 1998.

[0179] In addition to the films, coatings, and additives noted above,the optical materials of the present invention may also comprise othermaterials or additives as are known to the art. Such materials includebinders, coatings, fillers, compatibilizers, surfactants, antimicrobialagents, foaming agents, reinforcers, heat stabilizers, impact modifiers,plasticizers, viscosity modifiers, and other such materials.

[0180] The films and other optical devices made in accordance with thepresent invention may be subjected to various treatments which modifythe surfaces of these materials, or any portion thereof, as by renderingthem more conducive to subsequent treatments such as coating, dying,metallizing, or lamination. This may be accomplished through treatmentwith primers, such as PVDC, PMMA, epoxies, and aziridines, or throughphysical priming treatments such as corona, flame, plasma, flash lamp,sputter-etching, e-beam treatments, or amorphizing the surface layer toremove crystallinity, such as with a hot can.

[0181] For some applications, it may also be desirable to provide thefilms and other optical devices of the present invention one or morelayers having continuous and disperse phases in which the interfacebetween the two phases will be sufficiently weak to result in voidingwhen the film is oriented. The average dimensions of the voids may becontrolled through careful manipulation of processing parameters andstretch ratios, or through selective use of compatibilizers. The voidsmay be back-filled in the finished product with a liquid, gas, or solid.Voiding may be used in conjunction with the specular optics of theoptical stack to produce desirable optical properties in the resultingfilm.

[0182] Converting

[0183] Various lubricants may also be used during the processing (e.g.,extrusion) of the films. Suitable lubricants for use in the presentinvention include calcium stearate, zinc stearate, copper stearate,cobalt stearate, molybdenum neodocanoate, and ruthenium (III)acetylacetonate. In addition, the film may undergo subsequent processingsteps such as converting, wherein the film may be slit into rolls orfinished sheets for a particular use, or the film may be slit orconverted into strips, fibers, or flakes such as are used for glitter.Depending on the end-use application, additional coatings or layers asdescribed above may be added either prior to or after a convertingoperation.

[0184] The multilayer optical films made according to the presentinvention may be converted into glitter in any of a variety of desiredshapes and sizes (including copyrightable material or a trademark, e.g.movie or TV characters), including a registerable trademark orregistered copyright as defined under the laws of the countries,territories, etc. of the world (including those of the United States).The periphery of the glitter may be, for example, a regular,predetermined shape (e.g., circles, squares, rectangles, diamonds,stars, or alphanumerics, other polygons (e.g., hexagons)), or anirregular random shape and mixtures of at least two different shapesand/or sizes. The size and shape of the glitter is typically chosen tooptimize the appearance of the glitter or to suit a particular end useapplication. Typically, at least a portion of the glitter has particlesizes (i.e., maximum particle dimension) less than about 10 mm; moretypically less than about 3 mm. In another aspect, at least a portion ofthe glitter typically has particle sizes ranging from about 50micrometers to about 3 mm; preferably from about 100 micrometers toabout 3 mm. Conversion of the film into regular, predetermined shapes istypically done using precision cutting techniques (e.g., rotary diecutting). Conversion services are commercially available, for example,from Glitterex Corporation, Belleville, N.J.

[0185] The thickness of the multilayer optical film comprising glitteris typically less than about 125 micrometers, more typically less than75 micrometer, and preferably less than 50 micrometers, and thicknessmay go down to 15 micrometers for applications such as automobilepaint). Multi-layer films suitable for use in making glitter accordingto the present invention preferably have sufficient inter-layer adhesionto prevent delamination during the conversion process. The thickness ofthe film (in the z direction) from which glitter according to thepresent invention is preferably about 3 to about 25% of the smallestglitter particle dimension (i.e., measured in the respective x and ydirections). Preferably, the glitter is sufficiently thick to remainflat in application, but not so thick as to create substantial edgeeffects (i.e., distortions on cut edges of the glitter particles thatextend into a substantial portion of the film thickness).

[0186] The glitter may be incorporated into a matrix material material(e.g., a cross-linked polymeric material) in one or more subsequentsteps. In one embodiment the glitter is dispersed (e.g., uniformly ornon-uniformly) within a translucent (including transparent) matrixmaterial such that at least a portion of the glitter is observable by aviewer of the composite material comprising the matrix material and theglitter. The matrix material need not be translucent (i.e., can beopaque) provided that glitter is at the outer surface of the matrixmaterial such that at least a portion of the glitter is observable by aviewer of the article. The glitter made according to the presentinvention may also provide an article or composition comprising asubstrate, a matrix disposed on the substrate, and a plurality ofglitter disposed in the matrix.

[0187] Techniques for incorporating glitter made according to thepresent invention into the matrix material include those known in theart for incorporating conventional glitters into matrix materials. Forexample, glitter can be dispersed in a liquid, for example, by mixing orotherwise agitating the liquid with glitter therein. Dispersion of theglitter in the liquid may be aided, for example, with the use ofdispersion aids. In some cases, a liquid having glitter dispersedtherein is a precursor for a composite article derived therefrom. Forexample, glitter can be dispersed in a curable polymeric materialwherein the glitter containing polymeric material is placed in a moldhaving the shape of the desired final article, followed by the curing ofthe polymeric material.

[0188] Articles comprising glitter-containing matrix materials may bemade by any of a variety of techniques including cast molding, injectionmolding (particularly useful, for example, to make three-dimensionalarticles); extrusion (particularly useful, for example, to make films,sheet materials, fibers and filaments, cylindrical tubes, andcylindrical shells (i.e., pipe). Sheet or film materials may comprise asingle layer or a plurality of layers (i.e., a multiple-layeredconstruction). Multiple layer constructions may have the glitter in oneor more of the layers, and may optionally contain different shapes,sizes, and concentrations of glitter in different layers. Further, forexample, glitter made according to the present invention may beincorporated into, or mixed with, polymer pellets suitable for injectionmolding. Other examples of processes for incorporating glitter accordingto the present invention into a matrix material of a finished articleinclude vacuum molding, blow molding, rotomolding, thermoforming,extruding,, compression molding, and calendering.

[0189] Articles incorporating glitter made according to the presentinvention may, for example, have the glitter uniformly or non-uniformly(including randomly) dispersed therein and/or thereon, as well have someareas with the glitter uniformly or non-uniformly dispersed thereinand/or thereon, and other areas wherein it is non-uniformly oruniformly, respectively, dispersed therein and/or thereon. Further, theglitter may be present such that there are concentration gradients ofglitter.

[0190] The present process may include the step of orientation of theglitter in the matrix material. The glitter particles may, for example,be random with respect to one another, or have substantially theorientation relative to one another or relative to a surface of thematrix material. Alignment or orientation of the glitter within thematrix material may be provided, for example, by high shear processing(e.g., extrusion or injection molding) of glitter-containing matrixmaterial which results in orientation or alignment of the glitter alongthe flow direction of the matrix material. Other techniques fororientating the glitter within a matrix material may be apparent tothose skilled in the art after reviewing the disclosure of the presentinvention.

[0191] The glitter may also be randomly or uniformly distributed overthe surface of an article, and can be random in some areas of thesurface and uniform in others. Further, for example, the glitter can berandomly or uniformly (e.g., uniformly spaced) oriented with respect tothe surface, and can be randomly oriented in some areas and uniformlyoriented in others. The glitter can be patterned to provide, or be apart of, copyrightable material or a trademark (e.g. movie or TVcharacters), including a registered or registrable trademark under anyof the laws of the countries, territories, etc. of the world.Optionally, a coating (e.g., a clear coating) may be applied over atleast a portion of the glitter to provide additional bonding to thesubstrate, to provide protection to the glitter, or to provide a morevisually appealing effect.

[0192] Turning again to liquids having glitter according to the presentinvention therein, such dispersions, or dispersible combinations may besolvent-borne (i.e., dissolved in an organic solvent), water-borne(i.e., dissolved or dispersed in water), single component, ormulti-component. When the dispersions, or dispersible combinations areto be used to provide a coating on a surface, the liquid may preferablybe a film-forming material.

[0193] Examples of liquid mediums, although the compatibility (e.g.,chemical compatibility), and hence the suitability of a particularliquid will depend, for example on the composition of the glitter, aswell as other components of the dispersions, or dispersiblecombinations, include water, organic liquids (e.g., alcohols, ketones(for a short period of time)), and mixtures thereof. It is noted thatsome matrix materials may sometimes be liquids, and other times a solid.For example, at room temperature, typical hot melt adhesive materialsare solids, whereas when heated to their respective melting points, theyare liquids. Further, for example, liquid glue, prior to curing and/ordrying is a liquid, but after curing and/or drying, is a solid.

[0194] The dispersions, or dispersible combinations, may be, forexample, dryable, curable, or the like to form yet another matrix (e.g.,a paint may be dried or cured to provided a solid or hardened form). Thedispersions, or dispersible combinations, may include additives (e.g.,antimicrobials, antistats, blowing agents, colorants or pigments (e.g.,to tint, or otherwise impart or alter the color of, the matrixmaterial), curatives, thinners, fillers, flame retardants, impactmodifiers, initiators, lubricants, plasticisers, slip agents,stabilizers, and coalescing aids, thickening aids, dispersion aids,defoamers, and biocides) which provide, for example, a desirable featureor property in the desired final composite (comprising the glitter),and/or aid in the processing step(s) to make the desired final composite(comprising the glitter).

[0195] In one aspect, the dispersion, or dispersible combinationincludes binder precursor material (i.e., a material that is convertablefrom a liquid (i.e., a flowable form; e.g., polymers dissolved in asolvent, polymer precursors dissolved in a solvent, polymer emulsions,and curable liquids) into a solidified or hardened form. Processes toconvert a liquid binder precursor material to a solidified or hardenedbinder material include evaporation of a solvent, curing (i.e.,hardening via chemical reaction), and combinations thereof.

[0196] Additional examples of binder precursors and binders for thedispersions, or dispersible combinations, containing glitter accordingto the present invention include vinyl polymers, vinyl-acrylic polymers,acrylic polymers, vinyl-chloride acrylic polymers, styrene/butadienecopolymers, styrene/acrylate copolymers, vinyl acetate/ethylenecopolymers, animoalkyl resin, thermosetting acrylic resins,nitrocellulose resins, modified acrylic lacquer, straight chain acryliclacquer, polyurethane resin, acrylic enamel resin, silylgroup-containing vinyl resin, and combinations thereof.

[0197] Examples of dispersions or dispersible combinations, that cancontain glitter according to the present invention include fingernailpolish, paint (including paint for automotive and marine applications,indoor and outdoor house paint, art and crafts paint, hobby paints(e.g., toy model paints), and finger paints). Such dispersions ordispersible combinations, are typically applied to a surface to providea coating which is subsequently dried, cured, or the like to provide ahardened or non-wet surface coating.

[0198] The size, shape, thickness, and amount of glitter used in aparticular application, including applications described herein, maydepend on a number of factors, including the desired effect to beachieved, cost, inherent limitations of the application (e.g., if theglitter is in a binder material, the amount of glitter should not exceedthe loading capacity of the binder matrix, unless it is desired forexcess glitter to easily fall out), and for liquid matrices, theviscosity of the dispersions, or other physical properties orperformance characteristics of a matrix having the glitter therein.Glitter made according to the present invention may also be applied to asurface by first applying a binder or adhesive material, then applyingthe glitter, followed by drying, curing, solidification, or the like ofthe binder or adhesive material. Examples of substrate for adhering theglitter to include toys, fabrics, sheet materials (e.g., paper,cardboard, and films), ornaments, plastics, wood, and metal. Adheringglitter to the surface of a substrate can, for example, provide adecorative effect.

[0199] The glitter may be adhered to the surface using any suitable formof attachment, such as glue, pressure sensitive adhesive, hot-meltadhesive, and stitching. When adhered with adhesive materials, theglitter can, for example, be placed onto, or broadcasted over, thesurface of the adhesive-coated substrate. Placement of the glitterrelative to the substrate may be provided in any of a variety of desiredpatterns and/or orientations. For example, the glitter can be randomlyor uniformly over the surface, and can be random in some areas of thesurface and uniform in others. Further, for example, the glitter can berandomly or uniformly (e.g., uniformly spaced) oriented with respect tothe surface, and can be randomly oriented in some areas and uniformlyoriented in others. The glitter can be patterned to provide, or be apart of, copyrightable material or a trademark (e.g. movie or TVcharacters), including a registered or registerable trademark under anyof the laws of the countries, territories, etc. of the world.Optionally, a coating (e.g., a clear coating) may be applied over atleast a portion of the glitter to provide additional bonding to thesubstrate, to provide protection to the glitter, or to provide a morevisually appealing effect.

[0200] Additional processing steps such as are commonly known in thefilm processing art may also be used in the processing of coextrudedpolymeric multilayer optical films of the present invention. The presentinvention should not be considered limited to the particular examplesdescribed above, but rather should be understood to cover all aspects ofthe invention as fairly set out in the attached claims. Variousmodifications, equivalent processes, as well as numerous structures towhich the present invention may be applicable will be readily apparentto those of skill in the art to which the present invention is directedupon review of the present specification. The claims are intended tocover such modifications and devices.

What is claimed is:
 1. A method of making a textured multilayer opticalfilm, said method comprising the steps of: (a) providing at least afirst and a second stream of resin; (b) dividing said first and saidsecond streams into a plurality of layers such that said layers of saidfirst stream is interleaved with said layers of said second stream toyield a composite stream; (c) passing said composite stream through anextrusion die to form a multilayer web in which each layer is generallyparallel to the major surface of adjacent layers; (d) casting saidmultilayer web onto a casting roll; and (e) contacting said multilayerweb by a micro-embossing roll to form a cast multilayer film.